This is exactly the kind of exercise my buddy and I did while also doing what I described in my post above-- mastering the skill of hovering in one spot--but I wanted to keep the post short.If you've got another person to play with in the water, try hovering facing each other, then A hands off a 2-pound weight to B, who holds it for for a few breaths, then hands it back. The goal is for neither A nor B to go down or up in the water column...all with breath control. When you get that nailed, move to 4 pounds.
In our first pool session (after Fundies, but not relevant), we just tried staying still--not complicating it by attempting anything else. In later pool sessions, we added the sorts of exercises you describe and made some into games. It was a dive shop's pool, and we found a toy "missile" or dart on the bottom that divers could toss back and forth. Staying in one's chosen orientation (that could be horizontal trim or not) while task-loaded is the ultimate goal.