I want the headshot for my AR-50 but it's hardly a necessary mod. I think I'm just OCD at wanting all engineers unlockedCongrats. Very impressive.
I gave up on those some time ago. I think I have 4. The only mod that I can't get without that engineer is headshot damage, anyway, which is not something I care about in this game.
However, you don't need the Hero-->Wellington-->Uma line to get it if you're in Colonia. From what I can make out, it seemd easier to get that mod out there.