The entire point of my previously mentioned foray is to send a message. And by doing it loud and violently, it sends quite the message, particularly to anyone who may have thought we were cowed by our previous foray. Taking down a segment of the wall also prevents anything unfortunate occuring during an over the wall entry scenario, where even if we pick a point between two guardposts, if something goes wrong (ie an errant die roll), guards can be upon us in 1-2 rounds, perhaps leaving someone stranded in a vulnerable position. By bringing the wall down, anyone coming upon us has to fight on our terms. Bringing in a ShadowPhoenix squad (designed to be independent operators, recall) to back us up means more firepower in and around the entry point, along the path, at the objective and back again, giving us more freedom to engage in those targets we deem worthy of our attention.
I don't think there will necessarily be an ideal time to get the elves out. Although this plan is terribly risky and probably leaves much to be desired on the route home, the surprise and shock of such a raid could be much more valuable than a more highly developed plan. If necessary, we could have troops with wagons waiting at the woodline to help us escape or get the elves back to Hyrwl. If necessary the group + ShadowPhoenixes would run rearguard to delay/defeat any pursuit. We have shown no other realistic plan/creativity in attacking this problem and I have my doubts whether we ever will.
If the City Watch is deemed to dangerous, so be it, this method can also work for the Barracks, but I don't think we should get our hopes up that hitting the barracks will eliminate middlemen. We are likely to just be killing footsoldiers and maybe the occasional officer. I imagine officers are quartered in more comfortable abodes.
But maybe we're not ready for such a daring exploit.