I recall, I didn't say Mitsunari has overwhelming leadership than Ieyasu in the first place right? but Mitsunari has exceptional talents in administrating government, and in politics.
Yeah, Ieyasu did have better chances to beat Mitsunari, since Ieyasu has seen through some battle skirmishes before Mitsunari was placed in Toyotomi. It will be awfully awkward for Ieyasu to be beaten by someone who has no experience in war at all. Come to think of it, thats the idea right? to start an uprising against someone who know nothing to war, at a right time.
For the accusing part, Mitsunari has to be suspicious of Ieyasu. After all, Ieyasu is the only clan who fought against the Toyotomi, and gaining one of the promising power and military authority at that time. Mitsunaris worries is not without reason, as Maeda pass away, one of the strongest military support and pillar of Toyotomi, his worry for the Toyotomi Clan doubles. As Mitsunaris knew he can't be able to hold out Ieyasu, if he stays idle by to watch Ieyasu slowly working his way to consume Toyomi Clan in their weakest hour.