My question was theoretical. I was reading about the two amylases and just wondered what would happen if you let the mash temp drop through the full range.
I'm relatively new, and also use BIAB (where the final "B" is a stainless mesh basket.) For Belgians, I've been running the full temp range from the other direction - starting low, then raising mash temp in several steps until mash-out. I have read that with current "modified" malts, this is is totally unnecessary, but it works for the Trappist brothers, and why not? It's not more work, because I'm using a pump and electric heat. The step mash process is more repeatable than it was with gas, and I can use my basement without asphyxiation.
I've also been told that with step mashing, I should remove Carapils and other dextrin malts from recipes. I'm a little less sure what to think of this. If I substitute in more base malt, it seems to drive the gravity slightly higher, and I don't notice any difference in the head or foam. Maybe I'm not perceptive enough.
I think you are close in grasping the points above, but perhaps some further clarification could help:
The beta amylase once denatured, stops the work of breaking the chains of sugar, leaving only the alphas to do what they can do, which they can actually do more than betas and quicker, but they are helped by the betas on breaking down specific starch types. You can google this for more info and get a more technical evaluation.
As to Carapils, I use it for flavor (as a light crystal malt) rather than the misconception of aiding foam or mouthfeel (it actually is foam negative in effect).
But in the end, try it for yourself and reach your own conclusions - Cheers!