inventors create it
engineers understands it and can optimize it
mechanics repairs it
good question
I think an engineer can easily do those 3 tasks. Trust me, I'm an engineer.
No but at least how I see it, "inventor" is a side title kinda thing, you can be an engineer who creates things, so you're also an inventor. You can even be a mechanic who is also an inventor, or a cashier and inventor I don't know.
A "mechanic" I guess is more of the profession of fixing broken things people bring to you. The in-game engineer on the other hand can fix, play firefighter, rebuild and fire guns or piloting the ship if needed. My point is that this guys seriously know their shi- and multitask their way to death like a boss so that's why they aren't called mechanics...
... I think.