Any such purchases can only be made on select guns, which must be kept in their original, military issue condition. You wouldn't be able to mail-order a C&R to your house, then customize it as such without violating the law.
There is no such legal requirement.
Show me the Title 27 Part you're referring to....
C&R firearms that are modified in excess of what is allowed may no longer qualify for C&R status if/after they are modified. There is nothing "illegal" about it....
They just need to transferred as a non-C&R weapon.
07 FFL /Mosin-Nagant Custom Shop/Bent Bolts
Gun-Kote/ Cerakote Refinishing
Criterion Barrels/ Savage Re-Barreling