Hi LEW,
I don’t think the Dormier would make it any more difficult than any other type of protector that went over the topper and mattress. All of them usually have stretch sides and corners (like a fitted sheet) or elastic that lets you put it over the topper and then tuck it under the mattress. For most people “water resistant” would be fine and it would take a fairly serious “accident” to go though this but for “waterproof” then one of the membrane types is usually the choice even though it can be warmer. I chose the Natura because I wanted the wool but many others would choose the thinner type because it has less effect on the mattress but the most common type would normally be the thinner membrane types even with the possibility of some extra heat.
Lateral or “side to side” alignment as well as joint alignment can be as important as spinal alignment and can certainly be an issue. It’s discussed in a quite a few places in more depth including here and here and here and here and here.
The shredded latex in the Seven Comforts is relatively soft and less resilient than a solid layer. With side to side alignment on your back then displacement under the bony prominences (hips, shoulders, and pelvic crest) isn’t as important and you may be better off with a firmer topper to “hold up” the center part of your body more. I think your own experience, judgement, and “educated” intuition would probably be the best judge about whether a relatively firmer or softer topper would be better because I can’t see you on the mattress or feel what you feel. In either case it certainly wouldn’t be as “risky” as memory foam because even the shredded latex would be more resilient and supportive than lower density memory foam.
Congratulations on your new mattress
Once you receive it I would wait a few weeks to go through any adjustment to a new sleeping surface and any initial softening and break in of the mattress before deciding whether a topper is necessary of what type would be best if it is.
Phoenix