Bob:

I am wondering if you are going in the wrong direction on this.

If you are going to fill in the mail slot, fill in the peep hole, and
even the lockset holes, only to have to cut down the door to the
correct thickness so you can skin both sides....

why don't you just buy a new door blank? The repairs to the door will
take a a few hours, then to thickness the door can be quite a chore.

When I have to skive down the door to get the desired thickness it
takes a long messy time to do it on just one side. I have never done
it to both sides just to save the core of a door.

Thinking of all the hours involved if the project went perfectly, I
know I could never sell it as a repair. I could sell it as a repair,
though.

Buy a blank, moritse, drill it and prepare for locks before you take
the old POS out. You can even finish it before final installation.
This is much easier than what you are describing, not as hard as it
sounds, and the technical degree of skills needed drops quite a bit.

You can use the old door as a template to help you locate hinge
location and to match any cuts made on the door to adjust it for
closure over the years.

Robert