If I perceive proper what you might be suggesting, I don’t suppose this assault is possible.
Assuming your inputs use the SIGHASH_ALL transaction dedication, it doesn’t matter whether or not you’re the first or final to signal the PSBT: your signature will solely be legitimate for a transaction that creates precisely the outputs that you simply signed for. When one other participant malleates one in every of your signatures, they may solely change the ultimate transaction’s txid (wtxid in the event that they malleate the signature on a segwit enter). Whereas a modified txid may very well be an issue within the context of a pre-signed chain of transactions, it won’t alter the result of the transaction itself.
