Faulty logic and lack of foresight are pretty canon for Ephraim, though XD

I find it more likely that Eirika would have lots of children in this (very specific) situation (and canonically, she is fertile) than having Ephraim marry, really.

As for the ideal king thing? That could be taken as either the perfect well-rounded king - fantastic father, loving husband, amazing ruler - or the black and white AWESOME AT SPECIFICALLY KINGLY THINGS way at the expense of everything else.
Personally, I think Ephraim leans more toward the latter.