Will a radiant barrier still help if I can’t get it under the whole roof?

Yes. Think of radiant barrier as shade for your home, the more the better. If you park your car in partial shade on a sunny day it still helps, right? Installing radiant barrier has a cumulative effect (partial coverage works) and most people end up getting between 70-90% of their space covered. Just install as much as you can as fast as you can, and don’t kill yourself on the last little bit unless that’s just how you are.