What are the malleability threat vectors when passing PSBTs to other actors?

If I am passing a PSBT (Partially Signed Bitcoin Transaction) between multiple people, and I pass on my signature for my UTXOs for given inputs, can someone take advantage of malleability and cheat me out of the outputs I'm expecting?

What are the full ramifications of this? What is the most I can be taken advantage of, and is there any defense other than using funding with segwit spends? Can you be specific on how exactly such an attack would occur, and what portions of the PSBT might a malicious actor pay most attention to

If I understand right what you are suggesting, I don’t think this attack is feasible.

Assuming your inputs use the SIGHASH_ALL transaction commitment, it does not matter whether you are the first or last to sign the PSBT: your signature will only be valid for a transaction that creates exactly the outputs that you signed for. When another participant malleates one of your signatures, they will only change the final transaction’s txid (wtxid if they malleate the signature on a segwit input). While a changed txid could be a problem in the context of a pre-signed chain of transactions, it will not alter the outcome of the transaction itself.