The switch column and row pins on J207 and J209 look nasty. Seeing those, and hearing that there were no more switch issues when everything was disconnected... yeah I'm pretty sure your only remaining issue is scatty connectors.
Give them a bit of a scrub with this - and if you're feeling really fancy, cleaning afterwards with IPA - and you'll have no issues for a very long while.
In summary this entire thread so far is a working demonstration on just how miserable connectors can make you, for the want of needing a clean with something that can actually clean oxidisation properly (e.g. that 'pencil) and for the ribbon case - just proper installation.