The bridges aren't really a problem, there are only two (Three if you count the One at Rhöneis), the one at Kingsbridge Is more or less impossible to reach with an army unless you are already in Varsilia (It has mountains in the North and the Deadwood in the West, which Is nearly impossible to go through i think).
As for New Garand position it's a bit stupid, but we know from the story of the veterans near Castle Storm that the Inori Desert isn't an easy place to resupply an army, and they were Varsilians, their Capital was near and they had a fleet to help them, i don't think syndarun (i don't think the name is correct) can be able to supply a Mercian Legion, thus making a siege or invasione pretty hard.
We should also Remember about the fact that Mercians have pretty bad relationships with both Ilmarans and Varsilians, which means that they probably have to keep an eye on both of them, thus not being able to mobilize all their legions against one country (not counting the fact that i suspect most of the Mercian army Is stationed on the front with the Ork Kingdoms)