Hi Shopper,
I wouldn’t “completely trust” anyone’s recommendations (including mine) when it comes to choosing a topper because only you can feel what you feel on a mattress and what works well for one person may not work nearly as well for someone else. These are just “informed best guesses” that will need to be confirmed with your own sleeping experience and as you mentioned if you are uncertain then a good return or exchange policy can become a more important part of your purchase decision.
If you do decide to add a topper then post #2 here and the topper guidelines and sources it links to has more information that may be helpful.
These are all preference choices. There is more about how a separate topper may feel different from the same layer inside the cover in posts #3 and #4 here and in this topic. There is also more about the tradeoffs between a stretch knit cover vs a wool quilted cover in post #6 here.
Deborah at diynaturalbedding is very knowledgeable about the products they sell and a phone call to them will be your best source of information about their cover.
I didn’t notice in my last reply that you were in Phoenix and you have some great options in the Phoenix area to do some personal testing so you don’t have the same uncertainties of buying something that you haven’t had the chance to test in person. The better options and possibilities I’m aware of in the area are listed in post #4 here (which includes SleepEZ of course).
Phoenix