I've been reluctant to jump in on this one.
Whether it's male ego, or what, I'm not sure, but we seem to have this "my way is best" theory that we just can't seem to get over.
Personally, I will not touch exterior body panels with a sandblaster. Inner structural pieces are usually heavier and have lots of offsets, etc. that stiffen them enough that any stretching is not noticeable.
The high pressure, (even at 100lb) from sandblasting can, AND WILL, cause the metal to distort to varying degrees. Sometimes you get by, other times, you're not so lucky. The best aproach is to stay away from it!! I've seen far more panel absolutely ruined by sandblasting than not. Most by DIY'ers using small hobby shop blasters. It's not a pretty sight, nor is it easy to repair.
As far as the metal being stretched only on the outer surface....hmmm....don't think so. Think of what happens to stretch the metal. It's been said that the heat of the process stretches the metal. We know this to be false, simply because heating metal causes it to shrink, not stretch. Besides that, metal needs to be heated in a localized area to about 300 degrees for this to happen. If sandblasting is getting the metal THAT hot, I've certainly never seen it. The principals are the same as if you took a hammer and wailed away at it. Only difference is the size of the hammer. The sand is like millions of tiny hammer blows per minute. The surface isn't all that's getting stretched, but rather the entire panel, throughout. I have no known proof, but I would venture to guess the surface is extremely hard and brittle. Ever notice how tough it is to get rid of the sandblasted texture with an 80 grit DA pad??
Lastly, my advice, without seeing pictures, would be to find another set of doors and leave the sandblasting to heavier operations. It was never meant to be an option for cleaning automotive body panels.
If you want to safely remove paint and rust, your best solution is to have them professionaly dipped by a company worthy of such a task. Redi Strip is the way to go, as they do not use an acid based agent to remove rust and paint. They use a heavy alkaline solution that doesn't re-activate once moisture becomes present.
Good Luck with your project. Worse case, I guess I'll have to make you a new set of door skins!!