Since you're also running an Electrum server together with Bitcoin Explorer,
You should consider opening your own topic with information about any other programs that you're running or one setting that fixes another may break the other.

Using the .cookie file should also work but Bitcoin Explorer must know where to find it if it's not in the default directory.
If you can't find env file (you must have run the program "as-is"), you can also use a command line option to set it.
Refer to their configuration instructions in their GitHub Repo, here: github.com/janoside/btc-rpc-explorer?tab=readme-ov-file#configuration

Yes, but it depends on what you mean by "inspect", if through simple open as text, even if the wallet isn't encrypted, the keys wont be readable.
If through "hex viewer" tools, it's possible with unencrypted legacy wallet files and master keys for descriptor wallets.
If encrypted, you'll have to rely on wallet.dat-compatible tools.

I wouldn't recommend it unless the wallet.dat is corrupted though since you can still use the instructions on an offline machine.
Bitcoin Core doesn't need internet to restore or load wallet.dat files to be able to export the master keys/private keys.

It doesn't convert, "Restore" simply creates a wallet-name directory inside wallets directory and copy the selected wallet.dat file there, then attempts to load it.
The only update that it'll do is to update the keypool to 1000 with the old keypool intact, it'll remain non-HD unless imported with an HDSeed.

There are other commands that will upgrade, import HDSeed or migrate (to descriptor) the old wallet.dat file.

That's a general term for the first transaction in the block where the coinbase (or the block reward) is included as input.
Not to be confused with the company with the same name which is usually written with upper-case "C".

You can use any blockexplorer and look for blocks, like block height 800000: blockstream.info/block/00000000000000000002a7c4c1e48d76c5a37902165a270156b7a8d72728a054
The first transaction is the so called coinbase transaction.

Yes, as you said in the OP:

32-bit has a maximum value of 2^32 or 4,294,967,296, so the range is from 0~4,294,967,295.
It has a relatively low max value compared to the average ASIC's hashrate but miners rely on "extra nonce" (in the coinbase transaction) or any other data in the block header so that they can start the nonce from 0 again.

That is mainly used in the very old version when Bitcoin Core didn't have multi-wallet support.
Something our friendly neighborhood AI chat bot would answer.
Now, it's not recommended because Bitcoin Core now has a native way of importing wallet.dat files like the one in my instructions above.
Additionally, it's not good to replace files while Bitcoin Core is running as it may introduce inconsistencies or even file corruption in some cases.

You can do so (for testing purposes only) by following the same derivation path used in the descriptor and using your master private key as "BIP32 Root Key" in IanColeman's BIP39 Tool.
You can set the derivation path in "BIP32" tab, Custom Derivation path; "m" represents that master private key, the ' signs or "h" (in Core) represent hardened derivation.

The address may be legacy but the WIF private key should match the private keys of your TapRoot addresses at the given derivation path.
You can confirm it by checking the address' public key (without the 02 or 03 prefix) in the address' descriptor in getaddressinfo <address> command,
while you're at it, confirm if the derivation path is the same as the legacy address' derivation path in BIP39 tool, if didn't matched, the entered path is incorrect.

BTW, is the descriptor actually "tr(xpriv/0*)" and not "tr(xpriv/0/*)" or others?
Check it by entering listdescriptors true command, requires walletpassphrase if the wallet is encrypted.

The prvKeys are derived same way as other address types, the derivation path may differ depending on the client.
The address generation however is different, here's the spec: https://github.com/bitcoin/bips/blob/master/bip-0350.mediawiki

Either the above or the correct password isn't provided, both will result with unpaid invoice.

But since you've mentioned that you didn't created those;
Have you at least tried pay the indicated address but failed to type the correct password or left it blank?

If not, someone has direct/remote access to your machine but (fortunately) do not know your wallet's password.
If yes, that's normal as I've explained.

First thing first, the error is caused by spending that local transaction's output as input of the transaction that you've broadcasted.
Since the network isn't aware of it, it will cause that "bad-txns-inputs-missingorspent" for spending a missing input.

Don't know how you got an inbound local transaction but it's mostly caused by having an unconfirmed transaction dropped from the selected server's mempool
or for outbound txns; created a transaction but didn't broadcasted it and used "Add to History" instead.
"Address Unknown" are just scripts that Electrum can't decode, you can try to tick "download missing data" in the setting icon on the top-right side of it to see if it'll update.
If not, it's impossible to tell without the actual raw transaction.

In any case, backup it first by opening the transaction (double-click) and using "Share->Save to file" menu; (in case you need to restore it)
so that you can safely remove it from your history via "Right-click->Remove" so it wont be used in your transaction.
Alternatively, you can enable the coins tab via "Show->Coins", then go to that tab and freeze that specific output through "Right-click->Freeze->freeze Coin".

This had me thinking, you have a "user name" and not a wallet ID?
AFAIK, the closest thing to a username in the old blockchain(dot)info is their "Identifier".

According to their old recovery page, here (archived, don't use): web.archive.org/web/20141117184219/http://blockchain.info/wallet/forgot-password
the only way to recover the wallet is though their support by "convincing" them that you own it.
But since the identifier is useless in the new version, in case of a successful identification, you may be provided with a WalletID or something.

Since the topic is about Sparrow Wallet, it's worth noting that he wont be seeing those options if he used RBF, those are Electrum-specific options.

In Sparrow, the user can freely edit the amount with lower, same or even higher value as long as there's enough UTXO to spend.
The recipient address can also be changed manually.
Basically "increase fee" (deduct/preserve) and "cancel (double-spend)" options in Electrum for "power-users".
Electrum just simplified those as wallet features for the common users.

Most of the time, it's automatically discounted from your spendable amount so since LNbits shows '0', your local reserve must be higher than your local balance.

Upon checking my channels in Electrum, that is actually the case.
So your local reserve and the remote node's reserve should be 5000sats both by default.

I don't use Thunderhub but if you can find an option to check your channel's "local reserve", see if it's almost as much as your local balance.
If yes, that's what causing it to be short on available balance since it needs to keep that amount for future on-chain fees.

What about the rpc credentials on your "btc-rpc-explorer.env" or ".env" file?
Or have you set the configs via command line interface?

Since you didn't set the rpcuser and rpcpassword in bitcoin.conf, you must point the ".cookie" file to the correct directory.
It's auto-created in Bitcoin Core's bitcoin data directory, in the custom datadir if you're not using the default.

Child key derivation uses HMAC-SHA512 function which doesn't leave any mark of the data and key to the result.
There's no way to tell that 'Address A' and 'Address B' came from the same seed, even if the third-party knows the addresses' private keys.

A bit off-topic but it'll be sent to another address on Wallet A which most of the case, to a "change address".
But that's mostly client-dependent so other clients (wallets) may not follow that standard.

Similar but that may be an unrelated issue.
Based from that info alone, I can tell that it's an issue with the "coin_index" derivation path used in testnet which should be different from mainnet.
Still need confirmation with testing though.

But with their "busy" developers (since the original issue I reported above is still not closed) and the issue being testnet-specific,
I wont make the effort to report this issue myself.

Go to the directory pointing to "settings.json" file in the error message in your previous reply.
Open it as "text" and see if you can make some edits to the contents and save without errors (but make sure to revert the changes after).

But the failed write attempt to settings.json is less likely the culprit of the random crash but most likely a side effect of the underlying issue.
Furthermore, if it's the issue to begin with, Bitcoin Core will immediately alert you that it can't write to the file during its start process and will not proceed.

Your log looks fine and seem to be cut off while running, leaving no errors.

The error message concerning the settings.json file will also occur if it can't write to or find the path where it should dump the temporary settings.
Or try to follow its instructions.

The range is too loose to be specific on Bitcoin Core, perhaps it's your drive failing at some point.
I'd suggest you to test your hardware with stress tools specially the SSD/HDD/RAM.
And check the drive for bad sectors.

Take note that Bitcoin Core pushes your drive to its limit unlike other programs.

Although you're exclusively connected to your full node in the virtual Linux server and prevents inbound connections,
that node may still advertise your IP through addr message or when it receive getaddr message from its peers.
But I'm not sure if any of your config makes a difference on how the remote node create the addr message though.

Ref: developer.bitcoin.org/reference/p2p_networking.html#addr

Yes, that's one way to make it in Bitcoin Core.
And in the later versions, it will even output a "Descriptor" that you can import to a descriptor wallet where you can manage the transactions and co-signing instead of relying on commands.

The source doesn't matter since incompatibility to send to different address types is mainly a client-related issue.

Transactions are locked with a script not in the private key(s), in this case that MultiSig address.
Different addresses are representation of different locking scripts so the generated MultSig address will be treated differently as it requires different unlocking script to be spent.

Privacy issues, it'll be obvious that the MultiSig address and those two SingleSig addresses are owned by two individuals just by checking any of their "spending" transactions.

The only security concern in that is the double risk of getting their funds compromised due to using the same private key to two use-cases, but that depends on how they handle the private keys.

Bottom-line is it's a bad answer to OP's question.

You see, he's asking if it's enough to re-establish the MultiSig wallet, if he followed the advice that the descriptor is enough.
He may count on it in case the seed phrases are gone.

As you can see, that exported descriptor wallet doesn't contain anything that can sign a transaction from either cosigner.
You can check it yourself that it contains "extended public keys" and no master private keys or seed.