I would definitely rank MH above AK on reliability, so that could be a backup option to the MI flight. Before you pull the trigger on the MI flight, contact SQ and tell them your hypothetical situation and see if you could use the SIN airside transfer desks to check-in/get your boarding pass for the flights to ICN and beyond, without having to go through SIN Immigration, Customs, and Baggage Claim. If not, then 1 hour 40 minutes to accomplish all this is still probably doable, as long as the incoming flight KCH-SIN is on time. Or go for the earlier, safer flight on MH.