Can you select these maneuvers twice per round, accepting the strain penalty? Would that upgrade difficulty by two, and upgrade dice to hit by two?

Well. As it doesn't specify in the rules - as far as I know - that would be up to the group really. I'd allow it, at least to see how it works.

You'd have to pay the Strain and System Strain of course, pluss as the benefits increase, I'd say the penalties increase with it. So for Stay on Target enemies will also gain two upgrades against you (if I remember how that works correctly now) if you use it twice.

You can do both. I remember one of the devs, I think it was Sam Stewart, talking about this on a podcast. There isn't much point in doing it, since you're upgrading all attack dice pools AND all difficulties, but you can certainly do it.

Kaosoe has this one in his Developer Answered Questions thread. The answer is, "Yes, you can." Keep in mind the System Strain from using such a maneuver twice in a turn makes it of questionable use.