Hi healthysleep,
Unfortunately, there’s no way I would be able to determine if any product would be “firm enough” for you, as you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you, especially with your unique considerations.
Your in-store testing can prove to be useful if you do decide to place an order with Sleep in Latex (as you mentioned, they are a site member here, which means that I do think highly of them). I would phone them and let them know of the exact configuration you liked in both the Naturepedic and the Savvy Rest mattress, and they could then approximate the appropriate configuration in their 9" mattress that they feel would best suit your particular needs. They would be familiar with both of those companies. And as you stated, Sleep on Latex does have a 100 day return policy.
As for mattress protectors, I don’t keep track of the current offerings that keep growing from these companies, but this post and the many posts it links to offers quite a bit of information about mattresses protectors, including zippered ones.
Phoenix