If you're right on the borderline between 2 packs with different capacities, I'd wait until after you have the new tent. However, if there's no doubt about the size of the pack, then I don't think it matters which you get first. In fact, if your present tent will fit in the new pack, I might get it first. You'll save more weight on the pack than you will on the tent.

As far as Sherpa trips: will the new pack be able to handle them (my guess is no)? If not, then you'll have to deal with the old pack for those trips (and, if it's been working OK so far, it should continue to do so.) You might be able to replace it with a pack that is the same carrying capacity, but a pound or so less - not sure it's worth doing; you can save more weight by the other things you've already mentioned.

I can't really speak to the temperature limits of alcohol stoves, as I have very limited warm-weather experience with them.