Will the Bitcoin network discredit a transaction if you do a simple transfer on a multi-sig after you found the collision on that address?
No, this is not a palpable concern.
Multisig is generally implemented by means of Pay to Script Hash (P2SH) addresses. Hereby, the receiver defines a script that they'll use to spend the funds, and forms an address from a hash of the script. Later, when spending, the original script is revealed and must be fulfilled. In the case of a 2-of-3 multisig, this will require two valid signatures corresponding to two of three explicitly stated public keys.
The only way a collision could be found would be if, hypothetically, a script that only requires one key happened to hash to the same address. It is generally considered infeasible to search for such pre-image collisions other than by brute force since cryptographic hash functions are one-way functions, though.