Firstly, let me say that I think Magic Loop is an amazing invention! I used it for knitting sleeves in the round and it was so effective, and so much nicer than juggling DPNs…
I haven’t seen the video yet, but in your first question, it sounds like you’re just dividing your stitches - so it doesn’t matter if you divide in the “midpoint” or not. It’s like dividing your stitches when using DPN, you could have 20 stitches on two needles and 5 on the third… it doesn’t matter because you’re knitting in the round so they all get knitted anyway.
For your decreasing question, do you want the decreases directly over each other, or “staggered” so one row’s decreases are in between the previous row’s decreases? I reckon it’d be easiest if you take a look at your previous rows while you knit, and slightly modify your pattern to suit, “on-the-fly”. Perhaps more or less Knit stitches between the K2togs, to match your lining up or staggering of decreases. They shouldn’t show a lot, anyway.
Hope that helps! I’ll go watch the video now