Ok, why would there number one priority be trying to save Gelle-Musselmond? Putting aside the political issue of having lost Daurstein, I don't think they will have the time to actually save them. Montelivet has already taken Musselmond, with Gelre being less than a weeks distance away. Considering the duchy is named after those two cities, I think they will be surrendering well before reinforcements could come if the next battle isn't lost by the 3rd. Also, they will not be able to get us marching from the Silver Duchy, baring a frankly insane time expenditure.
The Ravoille northern bank and the Silver Duchy are completely different theaters? There is an entire mountain range and deep forest between those areas, which makes any move from our part of the front towards the duchy incredibly difficult and highly unlikely. In order for that to happen, they would need to cross a major river, go onto the Via Peregrina path and march on Damterre, something that takes way more time than 2 weeks.
You might have a point with the reinforcement times, but I still don't get how any of this is an answer to, "They pin us, since of course then we'd be the one having to deal with a river crossing, and then they shank us (strategically, I'm not proposing they hold us at the river crossing and then twenty-yards downstream they ford, we could and would stop that)."
The solution to such an attempt would be to cross the river onto the North Bank and confront the enemy before they can get set up, and then go from there, but if we're doing that then what great defensive positions are we getting from setting up on the South Bank? And if you're someone arguing that crossing into the North Bank is needless and dangerous aggression and shouldn't be done without the permission of the Convention, then will you do that at all?
I'm a supporter of the North Bank strategy, but included in this is the possibility of retreating if we're faced with forces we cannot fight, to the South Bank (hence the described defense in depth.) This is something we could manage because we have scouts and we presumably also hold the bridge, so I don't think we can actually be forced into a fight on the North Bank if we're categorically opposed to it.
But if a South Bank strategy turns out to give up a key element and is forced to then move North across the river in order to keep from being pinned down and stabbed to death, then where's the advantage? It'd be giving up the positive of the South Bank (the defensive position) without gaining the positive of the North Bank (the defense in depth and ability to do things like threaten the enemy and scout.)
Because this is, I feel, a good point:
Presumably, taking the northern bank will allows to actually deploy outriders to gather recon of enemy movements north of the river. Something that the southern position wouldn't allow. It puts us in a position where we can intervene before or during a river crossing.
We're going to be considerably more blind if we choose South Bank, which seems like it'd give the enemy an advantage in putting us in a bind.