For anyone trying import a watch-only wallet without Taproot into Wasabi:
In the JSON file set "TaprootExtPubKey" to null and "TaprootAccountKeyPath" also to null.
It should be possible to recover your "lost" coins by manually specifying the m/84'/0' derivation path when configuring the taproot wallet. I haven't tried that though.
I reproduced your situation on regtest and I managed to view the "lost" taproot coins in Sparrow wallet. I would also be able to recover the "lost" coins but that requires a patch to Sparrow wallet.
First to view the "lost" taproot coins:
Download Sparrow wallet
In Sparrow wallet create a new wallet
Set "Script Type" to "Taproot (P2TR)"
Under Keystore click "xPub / Watch Only Wallet"
Next to "xpub:" paste the xpub you gave to Wasabi Wallet
Next to "Master fingerprint:" enter "04b2430c"
What you type next to "Derivation:" does not matter but just enter "m" since it requires this box to not be empty
Click "Apply"
Switch to the "Transaction" tab and your "lost" coins should be there.
Recovering the "lost" taproot coins (this method involves typing the seed into a computer, making the wallet hot):
Set the number of seed words you are using (12 or 24)
Type your seed words into the "BIP39 Mnemonic" text box (assuming that you have a BIP39 seed)
Scroll down to "Derivation Path" and select "BIP32"
Next to "BIP32 Derivation Path" enter "m/84'/0'/0'"
Copy the "BIP32 Extended Private Key"
Create a new wallet in Sparrow wallet
Set "Script Type" to "Taproot (P2TR)"
Under Keystore click "New or Imported Software Wallet"
Next to "Master Private Key (BIP32)" click "Enter Private Key"
Enter your xprv (the one you copied in step 6)
Click Import
Set the derivation path to "m/84'/0'/0'"
Click "Import Custom Derivation Key..."
And this is where you would click "Apply" but it is grayed out because of a restriction in Sparrow Wallet which would need to be patched out. (I can take a look at this later)
Then your "lost" taproot coins should theoretically appear and you should be able to send them.
There is probably a way to do this without making the wallet hot, but that depends on the type of cold wallet you are using.
I appreciate the thorough look into this. Yes, sparrow can load the watch-only wallet just fine, the coins are there. It prompts for a hardware wallet for signing, but since mine is on electrum (tails usb) I don't think this is an option. Cant spend from here.
I also attempted loading in the bip32 extended private key as per your instructions. It allowed me to apply this, but none of the addresses are the same as the watch only, and none of the utxos are there.
Is your seed even a BIP39 seed?
If you generated it using Electrum it is most likely is an Electrum seed.
When you restore your wallet in Electrum you probably go through "Standard wallet" -> "I already have a seed". When entering the seed, do you change anything in the "Options" or do you leave it on the default "Electrum" seed type.
And on a different note: You do not actually have to patch Sparrow wallet. Under "File" -> "Preferences" -> "General" -> Turn off "Validate derivations"
"TaprootExtPubKey"tonulland"TaprootAccountKeyPath"also tonull.m/84'/0'derivation path when configuring the taproot wallet. I haven't tried that though.