Just remember that giving it that big a space above the hole might mean that the air isn't forced to travel through the shisha at all, it just get's sucked in the foil and down the hole.
The coals 'migrating' habit is normal with hookahs that vibrate a lot. I know my MZ does it. The coals pushing to the center will weigh down the foil and make the draw tighter and harder. In my case, it's noticable enough that when it happens I know to move the coals out again. Normally by the time it happens to me, the coals are only moving because they've burned right down, and are nearly done anyway.
As far as double layering the foil needing more heat? I smoke with 3 ******* hookah naturals on my phunnel every time. Don't know if I could get by with less but it does the job for me. I doubt another .3 of a millimeter of foil will be the difference between needing 2 coals or 3.
EDIT: DO >NOT< PUT HOLES ABOVE THE CENTER OF THE PHUNNEL! Caps for emphasis
. It will result in thinner and less flavourful smoke. You're just inhaling air like that, and it won't help at all with your problem. It will only make it harder to notice when it isn't drawing properly.