Took a few weeks, but all parts have finally arrived!
I didn't feel like it initially, but I ended up desoldering every single switch to remove the PCB from its frame.
Desoldering ended up being a good idea because I found more soda residues that were not visible before, plus it just gave me the opportunity to thoroughly clean everything else.
Here's the side of the board I couldn't see:
Here's the backside of the board, now desoldered.
Here's the frame (which I'm considering stripping to repaint it if the board works):
Here's a close-up of the cleaned ALT area:
I decided to test every single trace and diode—much more in-depth than what I initially tested—, but didn't find any issue either.
I'm starting to think my issue didn't require new parts… but those were cheap and can always come in handy!
Now, this might be a bit of a tricky question to answer, but
how many (and which) switches would I need to solder back to the board to test the ALT key again? I know it works a bit like a grid, so just soldering the ALT key alone isn't enough, but I'd rather not spend time soldering more than necessary if the board is actually toast.
Again, any help would be appreciated!