I think I may agree with you about all in on the second time to me.. Since the majority of the cards that could show up on the river are going to be scare cards.

Hands:
a5 (straight to split): 9 ways
22,33,44,88 (set): 24 ways
99,TT,JJ,QQ,KK,AA (overpair): 36 ways
(i don't think the overpairs are as likely due to the preflop action)
56 (straight to be beaten by): 16 ways

flush draw: I don't think it is that likely due to all the min raises that utg and mp have a flush draw. But co certainly could. But I didn't know that at that point. utg could have a flush draw if he was aggressive at that point.

So at that point I should be expecting a set probably. My thinking was it was a lot to risk being outdrawn if I am only going to split if I am not outdrawn and I am not already beaten. But I guess splitting wasn't as likely as I first thought. Since looking at the possible hands there is only 9 ways to make a5 and their are 6 ways to make each set. The 56 is somewhat of a threat. Probably utg doesn't have it since I wouldn't have limped with that utg. I wouldn't have called in MP either. Maybe I underestimated the chance of sets and overpairs here.