It aint rocket science!
Like anything else, some knowledge ((the link to the manual I posted isn't the only one out there -- all are similar).
All you need is time, a few tools (driver, belt with driver loop, shears <angled handle so your hand doesn't ride along the sharp just-cut-edges>, tape measure, marker, square). And a couple of ladders (I prefer 3 legged electrician type). Nothing hard.
As another person posted, it would be great if they built square and level. Makes installation much easier.
The door metal can be taller than the opening, as you adjust the hangers from the inside. Use stabilized wood for the frame (dried, or even a laminated type product). Wife took out the door hanger on her barn last year with a tractor (doors and gates around here are also called 'tractor magnets'). I had bought a 2x6 to replace a piece of the door (she was sure I was <still> WRONG on that one). Finally fixed the twist in it. The nice part about it was the wood was dry (I bought it the year before, but needed an excuse to bring it to the top of the list).
You may want to query the inspector on the legal process to get a contractor to repair his work == and check to see about his insurance and when it ends and how much it is worth. My Mom finally finished that with a bum contractor. Fortunately she found a good one. And was able to recover some of the costs (took a year; and only got 10~20%).
Bummer pics! You can't see all the issues, I'd bet, due to the insulation. If you take of the siding, something to consider are the see through siding panels. My shop has them in a 2~3 ft strip around the top (older versions look like opaque fiberglass; newer looks like clear plastic). Adds a lot of indirect light around the top of the building.