Free float handguard vs non free float: a few questions

[Wondering Beard]

I understand that a free float handguard can enhance the user's accuracy (not the accuracy of the rifle itself) because it doesn't contact the barrel and thus doesn't affect it when shooting.

So it is safe to assume that whatever pressure the shooter applies on the free floated hanguard, be it by tightening the sling or resting it on something or even by adding and/or replacing accessories mounted on the handguard, the rifle will always shoot to where it was originally sighted.

However, in the days before all the free floating rails, I believe the Marine Corps was training its guys to make hits a distances of at least 300 yards (I don't know what the basic skill requirements are for a new Marine). As a result, I'm starting to think that the advantages of the free floating rail may not be as great as claimed.

Thus my questions:

More or less how much pressure needs to be put on the rail to get a significant shift in POI. I don't need numbers but, for example woukl resting it against the side of a barricade, the way we see competitive shooters do, make a significant change and by how much would that be? would resting it on sandbags or even just using the magazine as a monopod while prone create the same problems? less?

If the shift is significant, is it predictable? in other words, if using the barricade method described above, can I just change my POA by x inches (assuming I already know what the shift is going to be) to compensate for the expected shift? or is the shift not that easily calculatable?

In a combat situations, has the use of cover to rest the rifle been known to create problems? if not, why not?

These questions are not about rifles whose main purpose is to make tiny groupss at long dstances. I'm talking about carbines that one would use to defend oneself and while I know that a defensive shot with a carbine is likely to never be beyond 100 yards, it would be good to know that a 300 yard shot could still be made, while using cover with a rifle whose handguard doesn't free float.