Regarding the inclusion of other monsters, I think that Ghidorah should have been omitted in favor of Titanosaurus. It would have fit the elemental theme better (Baragon = land, Mothra = air, Titano = water), plus - and most importantly - it would have given Godzilla someone to fight against physically. None of the other monsters were able to brawl with Godzilla, which took away a fun part of kaiju battles.
BTW, I'm a huge fan of King Seesar (I have a tattoo of a shisa), and while I'd have been thrilled to see him get some love, I agree that he'd have been out of place in this movie.
I'm not a hero. I'm not a savior. Forget what you know.
I'm just a man whose circumstances went beyond his control.