It's probably simpler if you'll utilize Ledger Live's "Accounts": support.ledger.com/hc/en-us/articles/4404389482641-Add-your-accounts
Use separate accounts for various use-case; e.g.: one for small transactions with small balance and one for large transactions.
When sending coins, you can select the account to spend from in the drop-down menu "Account to debit".

And in the coin selection strategy, use "Minimize fees (optimize size)";
that will keep the transaction size small as much as possible and in the process, Ledger Live will most likely select a single input.

Of course, even with those set-up, you'll still need to check the created transaction if there are inputs that you do not want to be linked.

Yes, either one of the two is safe for your privacy....

In this scenario, the transaction will use both as inputs, practically linking them together.

Yes, there's the "Coins" tab in in Electrum where you can select specific inputs.
In Ledger Live, there's also a "Coin Control" feature: support.ledger.com/hc/en-us/articles/360015996580-Using-Coin-control

Fee is mostly based from the transaction's size, more inputs & outputs means higher size, select one and it will be cheap.
In terms of anonymity, selecting two of more of those inputs will create a transaction that'll use those as inputs; thus, it will link them together.

I only took a glimpse on the previous post and I take that you prefer a single-key wallet?

If so, you can create a new "blank wallet" and import a single private key into it (it will derive 3 address types from that single prvKey).
e.g.:
That way, it will neither create new addresses nor change addresses.
However, you wont be able to use sendtoaddress since it requires a change address,
you'll have to manually create the raw transaction using createrawtransaction which involves precise computation from inputs' to outputs' amounts
and manual selection of inputs.

References for the commands:

• https://bitcoincore.org/en/doc/22.0.0/rpc/wallet/createwallet/
• https://bitcoincore.org/en/doc/22.0.0/rpc/wallet/importprivkey/
• https://bitcoincore.org/en/doc/22.0.0/rpc/rawtransactions/createrawtransaction/

Take note that receiving multiple transactions and change through a single address don't automatically consolidate all those coins.
Those will still generate separate UTXO per transaction.

Aside from the above;
If you're using lightning, using "swap" or "reverse swap" costs an additional fee on top of the on-chain transaction fee to pay for Boltz's service.

It's true, but not in that scenario (are those the actual words from the source?).
The attacker could come across a different private key that produces a "scriptHash" that's exactly the same as your MultiSig's scriptHash.
In that case, he can use his own "redeem Script" and prvKey to spend your funds.
It's about the "collision" explained by BlackHatCoiner.

If we disregard the collision of the scriptHash, just base it from your question above:
If the attacker came across your private key and want to spend the funds of the MultiSig setup, then he needs to come across the cosigners' private keys too.

Great but I didn't expected that to work since p2wpkh @ m/0' was already checked when you've used "Detect Existing Accounts".
I guess you only have bitcoins in a "change address" of your BRD wallet's Segwit address, because it wasn't checked by that button.

Anyways, congrats.

No, Electrum can't guess what derivation path to use in order to derive that address.
Addresses don't contain info about their derivation path.

Since it's bc1, you can try to restore another wallet by selecting "native segwit (p2wpkh)" then edit the derivation path below into m/0'
But I highly doubt it's there since it's included in the list of paths "Detect Existing Accounts" check.
The standard paths are also there (the 3 options without editing the derivation path),
and since there's only one result, only legacy " m/0' " has transaction history.

You mean it's among the newly derived addresses?
Then the balance should be updated after it appeared in the 'addresses' tab, check the history tab if more transactions appeared.

If 300 isn't enough, you can increase the amount of addresses to generate by changing "300" into any value depending on your BRD wallet's history.

Are you sure you're talking to a legit Trezor support?
The instructions seem legit; in fact, similar to my suggestion... It's just too weird that they are calling Bitcoin "bitcoin token".

Electrum can only support one account per wallet, in your case, it's your 'BCH account 1' restored to a Bitcoin wallet.

"Not enough funds" could happen if you use "Max" but edited the amount after.
That can also happen if you've "frozen" the input, check the "coins" and "addresses" tab if the one with balance has blue-highlight.
if so, right-click on it and select "Unfreeze Coin" (if the highlighted part is the 'output point') or "Unfreeze Address" (if the highlighted part is the address).

Unfortunately, derivation path is specific to the wallet.
AFAIK and as others have said, BRD used " m/0' " which is the path for "Non-standard legacy (Account 0)".

You can try to generate more address to see if your bitcoins is in your addresses higher than Electrum's default gap limit.
Enable the console tab (View->Show Console) and open it, type:
And for the change addresses:

Those will force Electrum to derive 300 more addresses past the gap limit.

This (testnet):

It shows an example where it detected two "accounts" with transaction history.

If there's only one result, then there's no other derivation paths detected and that was the sole derivation path with transaction history.
To go further, you can try other derivation paths that's not listed in the link in my reply above
but I have no idea what other paths BRD wallet used.

Then in the previous reply, you mean you've selected "Non-standard Legacy (account 0)" then it auto-selected legacy and auto-edited the derivation path.

How many results did you get? Just that one?
If that's the case, then Electrum couldn't find any bitcoins from all of the listed paths in the link in my previous post.
There's none in the second account index of that particular path either.

This doesn't look like the result of "Detect Existing Accounts".
The results of detect existing accounts shouldn't have the derivation path " m/0' " indicated,
instead, it should either be one of the "description" in this list + the account index: github.com/spesmilo/electrum/blob/master/electrum/bip39_wallet_formats.json
e.g.: Non-standard legacy (account 0)

Anyways, if there's a list after using the automatic account detection, then there's more than one derivation path that contain bitcoins.
In that case, restore as much wallets as the results.
You can restore the other accounts by using the menu "File->New Restore" and select the other path for the new wallet.

Since it's originally a 'BCH account' restored to a Bitcoin [BTC] wallet, there shouldn't be any other input aside from the wrongly sent one.
If you use the 'send' tab, it should automatically select that sole coin as input, you can now send it back to your BTC account.

But if you want to see the available UTXO, just enable "coins" tab: "View->Show Coins".
To select an input, right-click on it (you can multi-select with Shift/Ctrl+click) and select "spend";
a green "coin control active" note will be visible below and will use the selected coin(s) as input(s) when you send.

Ah, so what you meant in the previous reply is when you select "import bitcoin addresses or private keys"?
If that's the case, then it's normal.

If you can remember, the other standard wallet that you've created asked for a password in the last step of the install wizard.
Any wallet type is the same, the password will be prompted in the last step.

Then you either missed the last step where it prompts you to enter a password or you're using a fake Electrum
since the Desktop version doesn't have "password unification" like the Android version.

Are you getting any error message when spending the last batch of inputs?
If 20 inputs have worked, then 15 shouldn't be a problem.

Also, try to enable logging and see if there's an error related to the issue: "Tools->Preferences->Write logs to file", restart Electrum.
Reproduce the issue in that session, then you can disable logging again; the logs will be saved in "logs" folder inside Electrum data directory.

It's probably easier if you setup Electrum to use Bitcoin Cash's derivation path to access your BCH account's public keys.

Just follow this instructions from Trezor Wiki: wiki.trezor.io/Apps:Electrum
But during the step: "4. Choose the type of addresses in your wallet account.", do this instead: select "legacy (p2pkh)", then edit the derivation path below
from m/44'/0'/0' into m/44'/145'/0' or m/44'/145'/1' for the 2nd account... m/44'/145'/2' for the third and so forth.

Wait for a few minutes to sync the balance, then send the bitcoins back to the correct address.

That's normal if you're using the Android version;
That's not normal if you're using the Desktop version.

So it seems like you're using an Android device to claim your Casascius coin's bitcoins... that's concerning.

How about if N-days have passed and the puzzle isn't solved yet, open that round for all ranks instead of just newbies?
You can decide on the number of days.