still just as vulnerable to the Finney attack, unless you require your MAIT to reject new *blocks* that double-spend the transactions they accepted. And this introduces a risk of a fork. So, no.
As the OP said, the MAIT would reject new blocks that double-spend transaction they accepted. That's the only way for the fork to occur. If the MAIT has >50%, then it won't be a hard fork. Therefore, the MAIT should only reject double-spending blocks if the MAIT has >50%.