RE: Am I right about the mast foot position all the way back on an F-type?
If the wind has huge gusts (more than 5 knots higher than the ambient wind) you may be simply overpowered. That requires a change to a smaller rig, or some serious tuning of your existing rig to depower it.
If you can't sheet in, you can't put your weight on the rig.
With no weight on the rig, 2 things happen.
You don't get any mast foot pressure, and your weight goes right on the upwind rail.
So, the board does precisely what you are telling it to do.
No mast foot pressure to drive the nose down, causes your board to turn upwind.
Weight on the upwind rail tips the board to windward (windward rail lower than the downwind rail) and the board follows the rockerline upwind due to the shape of the bottom.
Flatten your sail out (more downhaul and more outhaul to reduce it's power and increase the twist at the top of the sail) and you may be able to sheet in OK, and then you can commit all your weight onto the rig, via the harness lines, and get your weight off the upwind rail and back onto the rig, driving the board through the mast foot.
Then you can use your legs and feet to adjust the roll angle of your board so you go straight ahead, on a beam reach; upwind by lifting with your front foot and pushing across the top of the fin with your back foot; or down wind by unraking your sail slightly and pushing with the heel of your front foot to "push" the nose of the board off the wind.
Hope this helps,