I'm not sure about making Yui an actual account (I think she could adjust her settings and match the lowest level and gear amongst the group), but the tree idea has some merit. So the 2 (Leafa and Asuna) who last the longest gets to baby the two that don't? Sounds good.
And as for the actual diapers in the game, Yui can simply use her admins access to create some (although she'd have to first research what it is as I doubt Yui would instantly know.
And for Leafa and Silica, is there half of the bet going to extend to real life as well or stick to in-game?