Andrew R Stewart 

H2O bolt- You could continue trying to extract it (although my experience with extractors is that if the bolt is frozen in the threads an extractor is often a no go). You could drill out the bolt, trying to not nick the boss, then retap the boss. You could remove the boss completely and replace with a Rivnut one (although i hate them. Even with a good epoxy bond they can break loose in time). You could leave all as is and use a cage with a tab then just hose clamp it in place (this is the "on the tour" repair and why I use said cages).

Brake nut- How important is the brake to you? If you can access the center bolt through the unserside of the fork you could saw the center bolt in two. You could drill out the recessed nut. You could remove the caliper arms and such then cut/grind the front side of the center bolt away.

What ever you do for either repair, and for any future assembly, use lube on the threads of just about everything. When I had my shop in Cleveland I suggested this to every one who came in to buy parts. With a salt mine only a few miles off shore we saw a lot of rusted components. Andy.