Terms corrected thanks for the input. Do you have any suggestions for the question asked?
I was mainly trying to understand your question . . . still not sure. I'm not interested in correcting your terminology -- just trying to understand your question.
You're saying 2x12 "STUDS". A 2x12 STUD would indicate that your wall is 12" THICK. I think you mean that you're going to use 2x12 boards on the inside frame of your door and they'll be FLAT against the wall. Is that right?
I thought I did offer some suggestions, along with a pic that might help you understand what you needed.
If you'd post a picture of your opening, both outside and inside, it would be pretty easy to offer options. Hard to say how to frame it without seeing what you have to attach to.
You need to wrap the door opening with 2x6 material. The weatherstripping will ultimately attach to this. What is available to attach this to? -- that's the main question about what you need to do. Your inside door frame will butt up to this or overlap it -- depending on how deep your wall is.
Your overall door frame isn't going to be just 1-1/2" thick. You'll have at least one more framing member behind those 2x12 boards. Overlap them in such a way that you can just screw through the face of the boards. If you're running 2x12 boards vertically on both sides of the doors, you could run them from floor to ceiling. (They still need another board behind them to fasten to and will join or overlap your door jamb lumber.) You could notch into the 2x12 to let your 2x10 header sit on the 2x12 and then it would get screwed right through the face of it into the board behind it.
You won't need to attach the "edges" of the header and supports if they both have another board behind them.
I think the garage door guy's advice about "not cutting the 2x12s" and concern about "not hitting a crack" is causing confusion. This really doesn't need to be that complicated. Take a look at the picture I sent and figure out how you're going to get similar framing on the inside of your metal building.
Let's see a few pictures. . .