Hey
i've done a pss280 and it also has a couple of glitches (which were there before i touched it) 1 being to do with the keypad. With the voices i can key a number in and it changes, but with the rhythm it doesn't you have to use the plus or minus to change the preset, i.e 1 at a time. Also when you put the slider to power off it goes off until it reaches fully off then it goes back on again, dunno what thats all about. Maybe the keypad could be a common flaw with 280/290 which are pretty much same machine apart from colour of buttons, i think.
re: keys not workin, try and trace back and see if you can get it to trigger somewhere else on the circuit, can i ask where you where fiddling about when the position of the dead keys changed? it is a tricky one with these boards they have traces on two levels so alot of stuff you cant get at and also why battery damage could be messy and very damaging.
I got some nice bends on it gies a shout if you need any advice-cheers