for the record, what exactly have you cleaned? Just the part on PCB? It's possible that it's about the elastic metal parts that are "glued" to the dome.

The definite test would be to assemble device without keyboard dome (nor keymat), open terminal or whatever, and just short a non-working key with any_conductive_metal. If it works, then you have your answer. If it doesn't then the damage is even deeper.

/Estel