In terms of foil I agree that once you are up and riding in principle you do not need a board, but if you are beginning riding it is really hard to stay up all the time, and a well designed board will bounce up where a piece of ply will nosedive.
That is so true, where you mount the foil to the board will have a profound effect on performance too.
waveboard deck: yeah, the zeeko board and mine also can be used as waveboards when going without the foil.
usually skimboards don't have a big enough scoop so i would not recommend it as a beginner foilboard deck. one more thing: flex is not wanted. old directional kiteboards might be a cheap alternative.
the center of the lift created by the foil needs to be exactly between your feet. if you use front and back straps the foil mount and strap inserts need to be at the correct position. if you use a frontstrap only you can vary the position of your back foot a bit to get the balance right. if the mounts are far off, you will end up eighter with a too wide or a too narrow stance. if you are going strapless foot positions are obviously flexible.
the guy in the video seems to be the typical beginner trying to find his balance. i can not see any hint for an error in the position of the foil.
if the foil changes the center of lift depending on boardspeed the the stabilizer is not timmed correctly. slight changes of the center of lift depending on course are normal.
if learning this foil board takes so much time that demo would not help any, I would think the manufacturers would offer the option of buying a complete foil package and also the foil and mounting kit only at a much lower price. used surfboards are pretty common here and yard sales have them for $10 very often. $2 grand is a lot of money for something that looks cool and I won't drop the money for it, even though it might be the most fun i'll ever have after I've learned it. To build a market for these, sell more at a cheaper price and make money with volume sold vs profit per board.
I spent last week riding the MHL Lift and an older Carafino foil in Cabarete. My take is the Carafino was somewhat heavier/slower but perhaps even easier to ride, the Lift beautifully designed and built, lighter (all carbon), very smooth riding and a bit faster. I'm getting a Lift. As to the usefullness of the board, well, you do need something to stand on, and it needs to be strong. Some guys are putting foils on the bottom of surfboards, but my feeling is that a surfboard isn't going to be strong enough to last very long in that configuration, and that a lightweight all-carbon board makes perfect sense. Right tool for the right job.