OK, putting the mast foot 1/2" from end of the slot should not be any sort of problem.
I do not understand why the rig would too close when your place your front foot 4-6" upwind of the centerline and 4-6" behind the mast foot.
If you place your back foot with the heel on centerline and your foot/toes on the other side (downwind) of center with the back foot about your shoulder width behind the front foot.
If that ends up putting your back foot "ON" the centerboard handle, simply slide your back foot back a little or place it in front of the centerboard handle. (Which ever feels most comfortable to you.)
Perhaps the reason you feel the rig is too close is because you are sheeting it in far too much for slogging/sub planing conditions.
You cannot rake the rig back or pull it in almost to the centerline of your board when you are not planing at a board speed that is faster than the windspeed (i.e. you are sailing on the apparent wind).
If you rake the rig back at all, with the CB down, the board will almost immediately turn upwind.
Actually with the CB down, you really need to keep the rig slightly forward of vertical.
(If you draw an imaginary line from the mast base to the tip of the mast, the imaginary
line with tilt slightly foward (giving you a bit more room).
If you sheet the sail in hard, and you are not sailing absolutely upwind as high as the board/rig will go without stalling, you are actually taking quite a bit of the forward drive out of your sail and converting it to sideways (downwind) force.
Like all sailing craft, "when in doubt, let it out" applies to your windsurfing rig as much as it does to any other sailboat.
When you are in sub planing mode, just sailing along, there's nothing at all wrong with a little bend to the arm.
Of course you were taught to keep the rig at arms length, but that's a "synthesis" that many instructional programs use to get you accustomed to holding the rig "way out there" to keep you in a Fig. "7" stance.
It's only synthesis if you are not leaning back to cantilever your board out to increase it's weight.
At some point you will get into a harness and learn to trust your sail/rig to hold your entire body weight to counter the pulling force of the rig.
Try to only sheet your sail in to an angle (with the board's centerline) that gives you the best speed for the direction you are sailing...... as you change directions, the sheeting angle needs to change also.
Also, if your board is tending to sail upwind, you need to move the mast foot more forward, not back.
Moving the mast foot forward does 2 things for you.
It puts a little more pressure further forward of the CB. This helps turn the board off the wind.
It increases the waterline length (when combined with you moving your weight slightly forward) which gives the downwind rail of your board more "bite".
"In this position the nose of the board really raised out of the water making it very clear that in strong winds you could wheelie! I simply sailed accross the wind changing from upwind to downwind. It was obvious that you need to take care turning downwind as the wind can really overpower you quickly"
The "wheelie" effect is your Rio M trying to plane. If you get this point again (and you will) raise the CB (to decrease the drag) and let the board accelerate onto a full plane.
You can steer pretty easiliy by tipping your board slightly. Tip the upwind rail down to head higher upwind, or use pressure from the toes on your back foot to tip the downwind (lee) rail down to head further off the wind.
Once your board takes off on a plane, I'm almost sure you will begin to "crave" planing conditions more and more. You will be "hooked" for sure.
As far as the sail "loading up" when you head lower (futher off the wind), you need to sheet your rig out as you bear off to prevent the rig from loading up due to the wind coming from too far back.
Open your sail and you will find you can easily "regulate" the amount of pressure and you will sail faster and more comfortably.
Kepp working on the suggested tacing method, I think you will find it works really well and keeps you from falling in the water.
Hope this helps,