But taking a forward position to defend gives us one vital thing, space. It forces any enemy army to win 2 battles (Daurstein and Martelnac), rather than 1. If we weren't extremly lucky, taking Daurstein would have been another battle against the western army. Space can be traded for time, and occupied land undermines the willingness of Norn to continue fighting, eventually forcing a capitulation.
Trading the city for time is at most going to buy us 2-3 days, which might end up being useful, but is not enough to give us a march action for example. Martelnac has a militia and trading it would only buy us about that long.
Having a fallback position is useful, but we already have a fortified fallback position in Antreville. Sitting at Daurstein for any significant period would be a mistake when we could be linking up with 6th army instead. If we can link up and we don't run into coordination or command issues we'd have enough force to decisively defeat Von Trotha. If we do run into command (Guillory refusing any kind of offensive) or coordination (6th having critically low drill) issues we'll probably need to spend march actions sorting that out.
I also don't think Daurstein is going to have a significant impact on the willingness of Norn to continue fighting either way. These little boarder cities trade hands frequently in war, the defeat of the Army of the West is likely to have a larger impact for example.
Like my vote doesn't actually rule out an assault, even if it might slow it down by a tiny bit. Nor does writing to the Assembly slow us down at all, because we're writing and then doing it anyway.
I'm not against asking them to surrender, I'm against any commitments that would leave us in Daurstein for any significant period of time.