RE: full speed
If you are planing, and completely hooked in before you get back to the footstraps, I'd suggest going a bit more slowly.
When you are hooked in, often yo have alot less sensitivity to the pressure in your rig, and normally when hooked in, you have the rig sheeted in to the point that your forward drive is reducing and the sideways force is increasing (unless you are still accelerating).
So, don't hook is so early.
Best thing to do when you are "underpowered" is to rig a larger sail that will be powered up.
If you don't have a larger rig, think about slacing off your downhaul a little and the outhaul as well to increase the draft in your sail, making it more powerful.
Often, sheeting in more, shuts off your power even more. Try easing your sheeting angle a little to see if there's more power (foreward drive). In fact, sheet out a little, sheet in a little, and look for the "best" sheeting angle for your point of sail and speed.
This will always be changing, and the old sailing axion "when in doubt, sheet it out" applies here.
You have to be aware of "acceleration" at all times when marginally powered.
Try to only do things that create improved acceleration, and try not to do anything that causes the board to stay at the same speed or to lose speed.
If you unhook, you may be able to pump your fin a bit, to accelerate your board, or you can "flutter pump" with the back hand to gain a little acceleration.
As far as weighting the rig, you can do that by "squatting" slightly, but drop your weight straight down, not out and away from the rig like you would if you were powered up and expecting the rig to support you.
As far as falling in backwards, what is your fear here?
That's the easiest way to fall in.
If your fear is being hooked in and trapped under the sail, try doing some "practice exercises" on how to unhook and escape from under your rig.
Do this in waist deep water and find someone to "spot you" (like in gymnastics) so that if you get trapped and can't get out, they can lift the rig by the mast tip and give you some breathing room.
Practice "clearing your harness line off your harness hook with one hand, then swimming along the boom, toward the mast, and out from under your sail.
Soon you will be able to challenge any fear of being trapped, and calmly use "muscle memory" to clear the hook from the harness lines and move out from under your sail.
Hope this helps,