If we go by the story I have suggested, then the start point would probably have to be like a month or two into the war to give Kantle time to develop airship tech and for news of this development to spread. In the meanwhile the war up to that point and probably for a while longer would be characterized by armies of sky riders clashing, maybe these are the higher classes, people who could afford mounts or in the sake of Eve, those who could tame them. 

I plan for Marren to be part of the upper echelons of Rosilian nobility, say the son of some well renown parentage or whatever. My idea for an adventure plot that would focus on the wild island would be that he desires the gem of immortality and to get there and have a good chance to get it he needs an adventuring team of some sort, and a way to get there. Hence come the desire to have an airship and a crew. To get this he sides with the factions that views them as a legitimate threat from Kantle and to counter it Rose would need a fleet of their own. However to convince the rest of the leadership for a full fleet would be near impossible so he proposes that they build one, let him captain it and sanction this expedition to the wild island maybe because of the hidden treasure and powers that reside there, maybe something there could make Rose completely safe against their enemies. This might be attractive because, even without their massive ground armies, Kantle could still field significant numbers of Sky Riders and Rose has only so many of it's nobility to lose before things start to get bad for them. 

The first strike idea is still pretty good and I can do without the airship right at the beginning, or ever. However the first strike will really need to hammer in the message that Kantle can still cause significant damage even without the road connecting them and thus give convincing enough evidence that, the longer this war goes on in the same manner, Kantle will eventually win due to sheer attrition alone. Thus spawns the search for another way to either defeat Kantle completeley or to make the islands perfectly safe.