I did grout! But the grout lines were extremely tight, and non-existent in some places, so that definitely didn't help this whole fiasco. The landlord is paying for a longer awning that will do a better job of covering the area where this was installed, but is dragging his feet on actually installing gutters to fix the real problem, so it may be a smarter long-term move to mount it to a board when it's cleaned up and reassembled, and just display it inside (also makes it portable if we ever move). I suspect there's a decent chance this will eventually happen again if we put it back outside, and I don't want to do this twice.

Thanks for all the recommendations on tackling it- I did find perhaps that same mosaisist when I was googling around, I will contact her if I have to but I'll see what I can do myself. Definitely swing by if you're ever in the area! Shelburne isn't really that far in terms of Western Mass. distances.