Hi Simon
See my post on the 7th. I think you are only 5 mm out, but follow my procedure in that post. Yes, you will need to glue the dowels in place . Possibly leave them slightly proud of the surface and sand them level. Yes, you will need a 3 or 4 mm drill bit for the earth wire. Protect the guitar surface with cardboard, angle the drill bit and drill from just below the top of the new post cavity into the control cavity. If possible use a drill press. Its the only way to be sure the holes are straight.