Hi levander.
Many protectors are not fully water-tight, the cotton protector will absorb moisture, but depending on thickness and the way it’s woven moisture can reach the mattress to various degrees. The case of a wool protector such as St Dormier focuses more on breathability and temperature regulation and if the water content percentage is larger than what the wool can absorb the wool can become moisture saturated in the area of the spill, and moisture can bleed through onto the mattress.
I do not have experience with any of the products you listed but you are on the right track with your research. Hopefully, someone that used any of these products will see your post and chime in. While both SafeRest and Coop’s large no or reviews on Amazon can be compelling I’d try to read between the lines of what the consumer’s point of focus is in terms of the benefit they are looking for. I’ve seen a few low star reviews mentioning that they got stains using this product which voided the mattress warranty. You are also correct that some reviews are amalgamated from similar products of the same company that is different or changed over time.
If it were me and if the product is returnable, I would test the 100% waterproof claims on a corner of the protector’s top surface. Place some absorbent fabric under it and a weight on top of the experimental surface to keep everything in place and to create some indentation in which spill can collect. Then I’d pour some colored liquid and leave it overnight. Beet juice may quickly reveal some waterproof weaknesses but it would probably be impossible to wash 
As you already noticed there are tradeoffs for different types of protectors. E.g. a thin membrane protector which is advertized as waterproof may still allow water vapor to go through … it will be less breathable and can add to the sleeping temperature of the mattress because it can’t disperse the water vapor as easily as wool or stretch knit cotton protector and depending on the materials in your mattress it can “negate” some of the breathability effects of the layers below it and add to the insulation effect instead of the ventilation. People choose these because they are thin and can have less effect on the feel of the mattress and can have a high level of protection against liquid spills but the tradeoff is that they can raise the temperature. You had the water-resistant type (not waterproof) but a more breathable mattress protector that is much more temperature controlling and effective for all but the worst spills would choose wool and then others who want something more breathable yet that has little effect on the feel of the mattress but that only absorbs moisture (from normal sleeping) but may not provide great protection against accidents (unless you remove them quickly) and doesn’t “resist” liquids would choose cotton.
Protectors typically surround the mattress and are quite thin whereas pads can be a little ticker and can alter the mattress’s comfort and feel. Post #10 here has more information about mattress pads, protectors, and the difference between them. Halfway through post #89 here there’s more about the pros and cons of different types of mattress protectors for those who want (or don’t) to affect the feel and performance of their mattress. You may wish to check out some of our members here who have good pads/protectors that they recommend using. Also have a look at different types of mattress protectors here and here .
I hope this helps a bit as you deepen your research.
Phoenix