How to Sign a Multisig PSBT?

I am trying to sign an incoming PSBT in custom software I'm building and I'm having trouble understanding how the signing process of a PSBT actually works. I have a Trezor where the xPub I used in the 2-of-3 multi-sig setup is actually the child of the main key of the Trezor (for example's sake, say the xPub is located at m/26H).

From my understanding, I can use HWI to sign the PSBT, but I don't know what information is necessary for me to provide and what the Trezor assumes. I think that the Trezor fetches the private key for each of the inputs based on the derivation path in the decoded PSBT, but I want to replace it with the correct path for my program, Ex: If the PSBT shows m/44/0/0/1/0, I need to replace it with m/26H/44/0/0/1/0 because remember; my xPub is not from the seed but rather one of its children. Also, I believe for multi-sig signing, I need to provide the other public keys used in the transaction in order to create the next script, but isn't that included in the PSBT? If not, is it even possible for me to get the public keys of the other 2 wallets if all I have are their xPubs? (Doesn't hardened derivation prevent me from finding child public keys therefore if the PSBT uses keys at, say, path m/44H/0H/0H/1/0, I can't generate the keys?)​

I would really appreciate any help/simplifications because at the moment, it's all very complicated to me. Overall, my goal is to simply sign a PSBT with my recovery key that exists in my Trezor. And, here's all the information I have:
- PSBT (created and signed by one key in BlueWallet)

• Derivation path from seed xPub to the wallet xPub (example: m/26H gets the xPub of the recovery wallet)

• The other two xPubs

Let me know if I need to make any additional clarifications.
Thanks.