So, given I imported two xpubs from a BIP 49 setup and one from a BIP 84 wallet, then used the derivation path BIP 48 to derive all the receive and change keys, have I just made a huge spaghetti soup that I won't be able to sign?
I have no means to test this but you may be able to sign by creating a new MultiSig wallet with the correct derivation paths to the provided extended public keys.
BTW, in Sparrow, you can't use the standard derivation path of BIP49 in a MultiSig setup by design.
That's strange, I currently use BIP 49 in a collaborative multisig 3 of 5 setup, and imported it into Sparrow with no issue. I was also able to form a 2 of 3 setup using the same xpubs and derivation path in Sparrow.
I checked and that safety check can indeed be disabled.
Maybe you have the safety setting "
Validate Derivations" disabled that enabled you to use the single-sig derivation paths to MultiSig.