Single strap rear seems to reduce spin out and makes it much easier to pull the board back under control when you do

Size 13 feet for me, and a fox 105. I find using the outer rear position much better for board control when powered up. Get out on the rail and get it flying. I find it easier to get into centre straps, but no way would I let that stop me from putting the straps out wide.

I honestly cannot see how the size of your feet affects anything? Its a balance of CoE, CoLR and sail etc etc.
For any slalom or freeride, back of back strap at LE of fin is good starting point, then your stance width is your height and some personal preference.

On factory boards, if I don't like it I immediately measure the stance width and it is usually wrong (for me). Then mast base from tail (relative to other boards and rocker line)

Right - now I see what you mean, you spoke of forward and back movement. You mean only lateral movement.

I reckon it balances out. Bigger person can load a fin more / use bigger fin and so having foot further out is better. Really its like an inch so strap size can achieve the tiny lateral movement you desire?

For me the Fox was born to be riden with a slight angle of heel, with the leeward side of the vee laying flat on the water. Only the inner strap position offers me the complete control of this angle.. and it makes the riding so fun ! I tried the four volumes with assymetric trims (one side out one side in) and the feeling was the same for each. I also noticed that the rare persons I know who didn't like the board were riding "on the heels", sat in the harness with the ouboard position.

I run my Fox 120 and other sizes with the footstraps in the middle inwards settings. I have size 12-13 feet. I have no problems going up wind or downwind.