Can the position of the rear bag change the point of impact at 100 yards? It comes down to if your rear bag is not 100% percent the same for every shot and it change the point of impacts by a couple of inches

I would say only if your rifle is horriable bedded. Or maybe loose in the stock.

I have seen bipods change point of impact so I guess a rear surport could I haveb't seen it.

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couple of inches

Resting your cheek on the stock with different pressures is more likely the cause.

Yes it will---go to a BR match and watch... I shoot BR and take great pains to settle the bags and to make sure that the rifle is 100% level and that the rifle sits on the rest exactly the same for every shot.

This is the way it should be done if you want to really see how your rifle is shooting. The more variables you introduce into the process the less successful you will be. It does not matter what kind of rifle you are shooting the bag placement process is critical but it depends on how accurate you rifle and ammunition are. The best processes that are correctly employed will not make a 1 moa rifle into a .5 moa rifle but at least you will know what the true potential of the rifle in question is.

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Yes it will---go to a BR match and watch

That's just good advice in general.

The point to be made is IF the front rest and the toe bag are aligned correctly, and the rifle is mounted correctly, the rifle will track straight backwards. Obviously, if you are twisting the rifle as you hold it or as you pull the trigger or if the alignment between the front rest and the toe bag is bad, then the rifle will not shoot it's best.
So it's really a combination of several things that must come together.