But basically, if I use Migrate on the 2013 wallet, then can't I just export what I need for the watch-only wallet using importdescriptors false -snip-
You probably meant: "
listdescriptors false"
Yes, that's if you already know which descriptors to pick, otherwise you may miss one with associated UTXO if you wont export all since it's offline and not synced.
Im really going to need to test with testnet coins but the thing is the wallet would be a modern version 28 wallet so I wouldn't encounter the initial migrate step.
You can download an
old version of Bitcoin Core
(
pre-descriptor), start it with datadir pointing to your preferred temporary test datadir.
Even if it's done in the same machine, that shouldn't interfere with your current setup unless you overwrite something by using the same directories.