The cost required to march an army is the exact same cost it requires for that army to sit at home and do nothing at all. (Except for a small bit of repairs you could save if the unit sits in city.) And who wants to sit at home and do nothing?
Five days' march is no big deal. We do that, and more, all the time on AT. And if you plan on looting while you're in the field, it's that much less gold you need to bring. A few battles in the field, and you have less men to pay, and less equipment to repair.
Sirion may very well sit at home and watch Perdan fight alone against Ibladesh. I wouldn't blame them at all. It's not like Perdan didn't flat out declare that Sirion was too big, and needed cut down to size. 
But I don't think that Sirion would sit at home and watch Caligus die, or stab Caligus in the back. Caligus and Sirion are pretty strongly allied. And Caligus did help Sirion break Fontan, helping to bring the northern war to a rather abrupt end. So I have a very strong suspicion that Sirion will send troops to help Caligus, even if they don't send troops to help Perdan. And in helping Caligus, they will indirectly help Perdan.
We do still have to see what transpires in Westmoor, though. Sirion has yet to finish up with them. I don't know what kind of terms Sirion would demand of Westmoor.
Yes, but at home you can rely on a steady income from taxes and the ability to use a bank if need be. Out in the field, loot is first come first serve, and will not be able to support an entire army. Unless you manage to break into a city, of course.
