I would probably clean and repack the bearing with the retainer in place. I'd think the spindle is more likely to wear than the cups. Never serviced a BB with the retainer but doubt that it's needed if you destroy it getting it out.
Did you have one of the originals? 9.5 mm sounds right for a 3/8" bolt. There are only two common thread pitches: UNF (24 threads per inch) and UNC (16 TPI). You should be able to look at them and tell the difference.
I'd be happy to source them and ship to you but as I recall USPS has a...
Every bar maker seems to measure differently, Nitto must measure center to outside. I will have to look and see if mine is marked 400 mm or if I measured it as so.
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.