Hi sheep123,
Thanks for taking the time to share your additional comments and feedback in such detail … I appreciate it.
I also agree with Tuft & Needle that you appear to be much more sensitive than most regarding the effect of some of the thinner mattress protectors on your mattress because most people don’t seem to notice the effect of some of the thinner mattress protectors you’ve tried.
I also appreciate the link to the study about dust mites. The results are interesting because all the participants were tested for dust mite sensitivity and were clearly affected by dust mite allergens and the study also made clear that the mattress allergy encasements reduced the dust mite populations in the mattresses relative to encasements that didn’t protect against dust mites and yet in spite of their clear sensitivity to dust mite allergens the mattress encasements didn’t have any measurable effect on their symptoms vs the unprotected mattresses. This is certainly curious and the only explanations that seems top make sense are that the increase in the dust mite populations in the mattresses weren’t enough to cross their sensitivity threshold, that it may take longer for the dust mite populations and the allergens they produce to build to a level that would affect them, or (most likely) that a single method of controlling dust mite allergens isn’t particularly effective unless it is part of a more comprehensive multi-pronged strategy. I also agree with you that other methods of dust mite and allergen control may be more effective than an allergy encasement. In any case I’ve added a link to the study in my comments about dust mite allergies in post #2 here … and thanks 
A mattress protector (vs an allergy encasement) may still be worth the tradeoff in how it affects the “feel” of a mattress for many people though because most people wouldn’t be as sensitive as you and because it can be removed and washed more easily than a mattress cover and can help keep your sleeping surface in a cleaner and more hygienic condition. It can also protect a mattress against stains which in most cases will void a warranty (including if there is any visible evidence of spot removal such as watermarks).
It’s great to see that you and your mattress are getting along so well!
Phoenix