On that triangular opening, I agree that the best solution is to cut a tight fitting triangluar piece and fit it in there. If the 90 is accurate, you can use a clear triangle to lay out the new piece slightly oversized, then fit with sanding. Put a couple of blocks inside to glue the triangular piece in nice and solid. If it's out of square, then it's you and the sandpaper until you get an exact fit. Use a small piece of the triangular stock to determine the set back of the supporting blocks.
I have the rocker switches in a 4 gang box in my living room, and they have tiny levers that run the dimmer built in. Nice looking setup and great to dim things down of an evening.
Overall, it is a really nice looking job you've done there. I don't blame you for not tearing it all out to get it square. If you get a chance, watch the Tom Hanks film "The Terminal", in which he builds some really creative structures from what's at hand. Your manle brings that film to mind.
The more I do, the less I accomplish.