Anthony
Firstly, wow you have put a lot of work into your list of questions! Great job and they are mostly valid questions that a builder should be able to answer.…

Depends on what you want to achieve - presumably you want the extra "sound proofing" of the heavier door.
If that is the case you'll be dissappointed since the sound rating…

I thought you were getting jiggy with the engineer….
no probably not worthwhile, pretty rare to reuse material unless it is new. It might save a few bucks but not…

How is the door supposed to keep the weather out?
How will it contribute to the energy rating, leaking air as it does?
The quality of manufacture is terrible.
I notice…