It’s now time to report back, to tell the results of my experiments and what ultimately worked, to “follow up”:
I found that any waterproof mattress protector of any type makes the mattress just a little bit harder. Anything that’s tough enough to block liquids also distributes any weight above it over a wider portion of the layer below. It’s not a terribly significant effect, but it is there and it is consistent. I tried various protectors, and even risked no protector at all for a few nights to find out what would happen. It was clear to me that “more is not better”, that two waterproof mattress protectors where only one was really needed was a poor idea.
I found the six-sided (i.e. “bag”) waterproof mattress protectors to be even more effective, to be the top-of-the-line model recommended by most sources, and to possibly have the desirable side effect of also blocking bedbugs. They do have one very significant disadvantage though: by foregrounding “the drum effect” they can make mattresses much harder. Stretched tight, they don’t have enough flexibility to allow the mattress to do its job. One compromise solution is to use a five-sided waterproof mattress protector instead. Another compromise solution is to use a bag that’s a little too big, typically because it was made for a thicker mattress. The goal is to have enough extra material to avoid tightly stretching so one gets the drum effect, but not so much extra material the sheets won’t stay on. Ultimately I settled on the “too big bag” solution.
I didn’t experiment with “membrane” type waterproof mattress protectors where the waterproof layer is part of a “sandwich” of other materials, because I’m very leery of them. It’s too easy to launder them at too high a temperature and damage the membrane, but not realize it. It goes back on the bed and one thinks everything is fine …but when it’s finally needed it doesn’t work.
I found that mattress pads had both a “harder” and a “softer” effect, partially canceling each other out, with only the rest of the larger effect remaining. The “harder” effect comes from the quilting, which can be quite tight and pretty stiff (don’t be fooled by the “hand feel” of a small area, which always seems soft). The “softer” effect comes from the embedded padding/batting. In my case a couple “good” mattress pads didn’t actually work for me, but what did finally work well was one that was very loosely made with “box” construction and containing a high-loft filler.
Neither “ventilation” nor “temperature regulation” were issues that mattered to me, because the effects of the window and the baseboard heater -both of which are only inches from my bed- completely swamp out any effects of the mattress or bedding. (They’re likely important factors for others though, and may militate for something different than what worked for me.)
As suggested I revised the inside of the mattress, and now it’s “just right”. On the top I added a 3" layer of ILD 18 Dunlop process of a natural/synthetic blend that I purchased from a different source. (I still don’t know how to tell whether a piece is Dunlop or Talalay by looking at it, but strongly suspect that what I previously purchased as Talalay is in fact Dunlop.) Next I retained two existing layers, 3" of ILD 22 latex, and 2" of ILD 28 latex. I removed the bottom layer of 4" of very stiff ILD 50 (!) foam, and replaced it with 1" of more moderate ILD 35 foam. (Previously with weight widely distributed over the bottom layer of very stiff ILD 50 foam, it in effect acted as a hard surface, so my mattress was really only 5" thick.) The combination of the softer top layer and the more compliant bottom layer got rid of all my pressure issues without introducing any support problems.