I'm guessing you are right. I know that in the early years Cannondale used 5/16" - 18 threads on the cantilever studs.
Were the M10 bolts too large in diameter? Guessing that the size is 3/8" - 16. You should be able to find them in the UK if that helps with shipping cost.
I'm not certain but I'd guess that the Black Lightning logo would stand up to lacquer thinner. If not I guess you wouldn't be out much if you end up having to repaint it.
I set the limit screws with the cable disconnected. Turn the crank with one hand and run the derailleur with the other. You want to adjust it so that it will shift to all cogs but prevent the cage from contacting the spokes.