Sorry I saw you mentioned Mouse Keys already, I’d still check out those articles, it would be very strange for those specific keys to not be responding. Especially if they work on the login screen.
If so, you can also create a new admin with a fresh set of preferences, or even boot into recovery mode, (Startup holding Command R, then select Utilities > Terminal, and type).
Chances are if it works everywhere but your user profile, I’d say a repair will not fix it, an you should look at the user profile instead.
But can you confirm if the same keys do work elsewhere? And also, even, if an external keyboard does the same?
Another good test, (While you’re in the Accessibility Settings is the on screen keyboard, how does that respond?). Cheers.