It sounds like the OP is concerned about receiving a non-typical transaction. Does the possibility exist that someone could perhaps create a transaction where the "Txout-script" is created in such a way that a signature from either of 2 different private keys would satisfy the spending requirement (sort of like multisig, but requiring either sig instead of both)? Could such a non-typical transaction be created in such a way to get it to show up with listunspent or listtransactions?
Yes.This is the question.It is possible to create a non-typical transaction with createrawtransaction command and it is possible to add a second public key to "Txout-script" which makes this amount can be spent by 2 different addresses.
I beleive this type of transactions also appears in listtransactions .