you could check to see if there is folder called previous system or incompatible software. some times it is a hidden folder. If there is a previous system folder then it is likely that you can recover then from there alternative if you have been using time machine you can find them there.
if you are able to do A Nvram reset then the same keys on the other side obviously work. since the KB encodes data of keys pressed then it is unlikely a mac fault and more likely to be the KB itself. I get this forth fact they one set of the same keys do what they are supposed to, I would look at cleaning the KB internal but that not an easy job so if it really bothering you the only answer I can see ai another KB. Make sure it is the correct KB for your model..
hi, repaired a couple of similarly treated imacs.. if you dorp your imac than the heat sensor on the hard drive may become detached then the the HD frys very quickly. A couple of machines just teh Hd read head tore into media destroying disk. basicaly a knocked over imac usually means new hard drive, upgrade the size of drive when you change and always have an external back of your data..