My opinion?
1.) Nobility or not, Crono and Marle--not to mention Lucca--would absolutely be on the front lines fighting for Guardia. Wild horses couldn't hold them back. It's not just Guardia's sovereignty and prestige at stake, but the lives of the people they care about.
2.) Even though, in real life, no human being could withstand the onslaught of an entire army, CML are not, by 1005 AD, mere mortals. They are the Entity's chosen ones, super-ultra-powerful warriors with magical weapons and the special gift given them by Spekkio. They would, if not WIN, then at least survive.
3.) Trying to puzzle out how Crono feels about royalty, blah blah blah is pointless. You really can't ascribe much to Crono when it comes to his opinions and motivations. What we DO know is that he loves Marle, Marle loves the kingdom, and Marle's family is now Crono's family. To my mind, any suggestion that Crono would not, then, fight on Guardia's behalf is absurd.
Personally, I think this topic is a pain in the butt, because you either think the one way or the other, and it seems like nobody EVER changes their mind about it.
OdF