repair process being done simultaneously during crafting
No. Cutting off sleeves is much more easier than repairing the trenchcoat and do not require any threads.
The nearly broken trenchcoat may have had most of its damage to the sleeves
This is just an assumption and obviously a bad one)
IMO too difficult to implement and not really worth it.
Just make a crafted item health equal to the most damaged consumable item health, IMO do not sounds so difficult.