There was some debate whether the previously-mentioned wall damage was from termites or water. I think we can now definitely confirm it was termites (and I suspect they enjoyed any dribbles from leaky windows!)

We pulled down the affected drywall and cleaned up the (non-insulated) space as much as possible. Our project-detour continued with the reinforcement of the disintegrating beams.

Fortunately, the structural beams were essentially unaffected by the termites. We were able to anchor the supports into the still-sound beams. The replacement drywall went in easily enough, although we still need to fill the seam between the existing metal window sill (which is basically an inverted U that capped the old drywall) and the new drywall (which wouldn’t fit under the sill cap due to wood and nails).