Apple - Does the MacBook Air draw power from the battery or wall socket when the battery is at 100% and plugged in?

Don't worry. It will use the wall outlet if that offers enough power which it should using the original AC adaptor.

What if the power is going from 100% down to 99.6%, then back up to 100% constantly?

That was until about a decade ago or so. Modern laptops even don't charge your battery if it is eg. at 98% for reducing unnecessary cycles. Don't bother about your battery in your computer.

If you want to care for your battery, just remember every cycle shortens battery life, no matter if 10 minutes on battery or completely emptied it. The battery has also some natural aging, so it will die after some years even if you never touch it.

I don't have an answer for you (and you should probably accept Ranon's answer, since your comment to that makes it seem like that's the answer you wanted). I do, however, have a test you can do to determine the answer for yourself:

  1. Check the cycle count.
  2. Leave it plugged in for several days straight (weeks would be better).
  3. Check the cycle count again.

If the cycle count hasn't changed, then it's not draining the battery at all.