The switch matrix is controlled by U10 the 6820/6821 PIA chip.
Since the wire has continuity from the switches back to J2 etc, have you measured continuity right back onto the actual PIA chip-pins too?
You may need to change the PIA chip, (which is a pain if it's not in a socket) as they can go faulty and lose function on just 1 (or more) pins, which might happen to be the row you're missing....this is often the case when wiring/connectors has been proven to be continuous from switch-to-chip-pins.
If it *is* in a socket, you may also have dodgy socket connections, and need to replace the socket too.
MM
