Can I use a larger PVC conduit and run multiple NMs through it (as a sheath from the panel to the top?
No
What is the fill for 12/2 and 14/2 in 1 1/2"?
Not important, because it is not allowed.
The end of the tube must be sealed - a really messy proposition - and you must staple all the cables within 1 foot of the top of the tube.
This was just discussed. You can put 9 nm 12-2 in a 1.5 inch tube as a raceway to the top. There is a 60 percent fill max. The end of the tube must be sealed - a really messy proposition - and you must staple all the cables within 1 foot of the top of the tube.
Why do this? Is there some requirement? Yes, it is allowed and it may be required but I would just screw a 1X6 about a foot above the panel and staple all the nm cables to that and forget the raceway.
A 1X6 is likely to split when driving staples, lumber today is of little value even though we pay top dollar for it. Would have been better to have mounted the panel to a plywood backboard.
Would have been better to have mounted the panel to a plywood backboard.