White Iris...couldn't Spock have just performed a mind meld?

Yeah, I don't think he could have since Kirk had suffered a serious blow to the head and therefore had a bad case of Hollywood amnesia (in addition to his hallucinations). Plus they established that Spock was having second thoughts (I won't say feeling guilty) about using a mind meld to erase Kirk's memory of Rayna. So it makes sense the way it played out in that Spock only used the meld in an emergency situation and could only see the "doors" (hallucinations) behind which the password was locked up.

A better question is why didn't Spock and McCoy relieve Kirk of duty before he had a mental breakdown on the bridge in front of the crew? He was obviously unfit. Of course there's precedent for them both acting and not acting under similar circumstances.