re ISSF rules
http://www.issf-shooting.org/default.aspx?mode=issf-sports&menu=22&mod=ifpg&pane=10&inst=6re NRA rules
I can't help w the NRA rules except to say check the NRA web site.
re target height
ISSF regulation target height is 1.4 meters (~55-1/8 inches) +/- 0.05 meters (~2 inches) to center of target from the level of the floor at the firing point.
This is easy to measure if the floor is level from the shooting position to the target.
Mine isn't, so I have to measure 1.4 meters at the shooting position then use a cheap Home Depot laser level to locate that height at the target.
re 3P
That is an NRA, 4H and maybe CMP match so you need to find/order the NRA sportster rule book. I had the rule book (for my own info), but of course now that I'm looking for it, I can't find it.
I've seen photos of shooters in the prone position on tables. I think this is to raise their position relative to the fixed target height, thus reducing the relative target height, so they are not shooting so up-hill.
re support hand
There are many different ways of supporting the rifle.
My arm put the rifle too low, so I have to use the knuckles of my upraised fist to raise the rifle. I have the back of my hand facing the target, and the rifle rests on the 1st 2 knuckles. And if you do not have a padded glove, it is NOT comfortable. In fact your knuckles will start to HURT from the weight of the rifle on those 2 knuckles. Once you develop your stance, with a heavy rifle like your LGR, it should be quite stable.
If you have "The Ways of the Rifle" you will see different shooters holding the rifle in different ways. Use what-ever work for YOU.
re removing the shoe
Should not be a problem as you are actually "removing" support.
re "drugs"
One general rule is anything that reduced your pulse rate is verboten.
Probably similar for anything that affects the muscles.
On the flip side, best not to drink coffee or coke before shooting either...cafeine will work the opposite way.