Help cutting/squaring floor joist for shed
Practice, practice, practice. Very few people are born with circular saw skills. 

Your blade guard issues might be a crummy saw, or you might not be coming in flat and straight. Make sure it moves smoothly and clean/lube it, if needed.

Wood being oversize is just one of those things. As long as you don't get less than a 10', you should be happy.

Regarding the undersized by 1/8" bit, you could probably ignore it, or cut all the rest of your pieces to match. Chances are you won't notice it either way.

If a guide board is working for you to get square cuts, then keep doing that. Otherwise, see the above about practice. For what it's worth, a sharp blade won't try to wander like a dull blade will.