The checksum rule is not always obeyed with all of the wallets. There is also still a chance that your mistyped or mis-copied phrases still matches the checksum even with the error.
It's more likely that you are actually restoring the wrong seeds. I can't think of anything if you have tried loads of deviation path and increased the gap limit.

CGminer was broken with Bitcoin Core's RPC when I upgraded from 0.19 to 0.20.0. The problem likely persisted till 0.21.0. Downgrading it back to 0.19.0 helped me to solve the issue.

I didn't try to fix the problem with 0.20.0 but that could probably be fixed by running a stratum proxy in between.

Can you get the master public key of the Trezor?

You can download a Electrum 3.3.8 to import the master public key and create a transaction from there, transfer it to an offline instance of Electrum 3.0.6 and then sign it from there before transferring it back to Electrum 3.3.8 to broadcast. Electrum 4.0.1 and above has incompatible raw transaction formats with older versions.

You don't have to try any other derivation path, just importing it back into MyCelium should suffice. You have to know both of them in order to recover the funds. Gap limits should be unchanged on MyCelium, derivation path should follow BIP44. Is there a problem with just importing it in MyCelium?

If it isn't there, then you might have the wrong seeds.

They shouldn't. Wallets are not supposed to transmit the seeds to any external servers. The entropy for the seeds should be generated on the computer, then no one else should be able to access your seeds.

Zero transaction fees are no longer feasible. It was calculated with the coin age as a factor a long time ago before it was removed. The inputs amount was also taken into consideration [1].

There is really no point to try to allow for free transactions to be relayed. The mempool is frequently being pushed to it's limits and it's fairly pointless to reserve any of it for free transactions; miners would rather choose the paid transactions.

[1] https://en.bitcoin.it/wiki/Miner_fees#Historic_rules_for_free_transactions

Looks like the mempool site went down for a bit.

The blockchair chart appears to be fine though. I'm not too sure about the updating interval but it looks like it dips to zero today which could be due to them not displaying the transaction count on 23/2/2021.

It has RBF enabled. If you want it to be confirmed faster, you can increase the fees by executing a RBF transaction to replace it with another transaction with a higher fee.

Your transaction is roughly 10vMB below the tip of the mempool with 77satoshis/vbyte. It can be confirmed in a day or two if the transaction volume drops. I would recommend you to wait.

What wallet are you using? Some wallets will rebroadcast the transactions till it confirms so it never gets dropped.

Are you using 0.00010934BTC/KB or is that the total fee paid? If it's per KB, then it could be confirmed within a reasonable period of time.

Export the wallet in the form of an unencrypted .key backup. You won't be able to synchronize the wallet, it's deprecated for a long time.

It's a MultiBit Classic file. You can download MultiBit Classic and load the .key file to restore the wallet, export it unencrypted and sweep into another wallet.

HCP made a post here about decrypting it without MultiBit Classic:https://bitcointalk.org/index.php?topic=5304446.msg55964982#msg55964982.

It's probably password protected. You need to export the file unencrypted for the private keys to be in plaintext.

You won't exactly know.

Typical CoinJoin transaction has many inputs and many outputs. The outputs usually have quite a few that are of the same amount. Chipmixer also does this, but they split it into many addresses with a common denomination. You'd want to explore Wasabi wallet, they have a convenient system for the coordinator for CoinJoin.

Is there any research done on physical attacks to exploit the OS or the wallet software to glitch it to reveal the seeds or any sensitive info? I don't think Electrum (in this case) can be immune to such bugs, I mean JSON RPC was unencrypted for a long time. Is it possible for the attacker to clone the disk/sd card to have access to the encrypted storage to bruteforce?
I don't think cold storage is insecure by any means. I've only switched to ColdCard because it is something like a cold storage while being way easier to use. It's slightly annoying to have to start my RPi up to make my transactions every time. Side channel defenses are just the icing on the cake.
I agree.

Both hardware wallets and cold storage are geared to defend against any traditional malware attacks and both can do so relatively well.

It's a stretch to say that cold storage is safer. Most hardware wallets are audited regularly, even at times by competitors who obviously have an interest to exploit each other's devices. Vulnerabilities can happen with cold storage as well, they are not immune to it. There are hardware wallets which acts like an air gapped storage as well, like ColdCard but with the added benefit of it being easier to use as well as it being hardened against side channel.

In terms of the theoretical exploitation surface, it could be argued that having a hardware wallet which is specifically designed for storing Bitcoins safely is better than a person with little to no knowledge having to set up one themselves and exposing it to unnecessary risks.

Open source ≠ free of exploits. Companies like Ledger operates like a corporation with specific NDAs to follow. If you don't like to use devices that are not open source, avoid them. Plenty of HW wallets which are open source still.

They do not invent attacks. Theoractical attacks are discovered with substantiated evidence that it is possible. I don't think it's bad to be researching on these to highlight the possible loopholes?

I don't think anyone goes as far to say that cold storage isn't safe. I've mostly seen people highlighting the focus of hardware wallets and the hardened nature of them against more novel attacks. If you are that conscious about security, then you could possibly get a hardware wallet shipped through a reshipper and do some simple auditing by yourself and could probably give you a peace of mind.

$5 wrench attack is much more likely and that is why most HW wallets have plausible deniability built into it as well. Don't think it's fair to shoot down hardware wallets like that; they do still provide much more protection against sidechannel attacks which is what you would be concerned about if you're absolutely paranoid. Using a drop address or a PO box for the shipping would be necessary as well.

If you do not have sole control of the private keys, then it is reasonable to assume that you were never in control of any Bitcoins at any point in time.

I don't see the point of the "miner" selling you the wallet without giving you any directions to retrieve it. What did you mean by Blockchain says otherwise? Did he give you an address or something?

As I've mentioned over and over again, if you are not capable of reading the codes, then you have no choice. Firmware codes are usually fairly long and it would take forever for normal users to attempt to audit them and probably won't even find anything wrong if there even is. I wouldn't go to the extent of blindly trusting them but I would rather find hardware wallets that are more used than those which are less known.

Ledger is not a great example; the problem is with their purchasing system but nothing is wrong with their devices that caused the leak. Auditing the firmware would definitely not prevent that, taking proactive steps to protect your privacy will.

I assume that you're paying the fees in the single digit per byte? When looking at the transaction information on Blockchair, do you see Replace by Fee: Yes or No?

Are you using TrustedCoin 2FA? Electrum 4.0.9 is working perfectly fine for me. Did you freeze any of the inputs as well?

You can try installing the previous versions here: https://download.electrum.org/.