Hi awoods,
It may be a good idea to talk with them first after some of your local testing so you can tell them a little more about you and your preferences and do a little bit of narrowing down about the mattresses they have on their floor that you may want to test. If you also try and meet with the same person you were talking to when you actually visit you would already have finished some of the “preliminaries” and you would probably be able to make much more effective use of your time when you were there.
Hopefully if you do some of the “work” on the phone then one trip will be enough to narrow things down to a “finalist” there but I think it’s very important for a couple to test mattresses together because having two people on a mattress can feel different and can identify some possible issues (such as motion isolation or “roll together”) that you wouldn’t notice if only one of you was testing the mattress.
Online manufacturers that sell component mattresses generally have “standardized” designs and firmness options that they use as a baseline to help their customers make choices that would have a high chance of success based on “averages” but I would keep in mind that as soon as a mattress you are considering has any significant differences (such as in the type, thickness, or firmness of the layers or the type of cover) then translating one design into another can be difficult and in some cases seemingly small differences between two mattresses can make a surprising difference in how they feel and perform in terms of PPP. If you test a local mattress that has the same type of materials or components in the same thickness and firmness levels and the same type of cover as an online mattress then they will generally be a close approximation but if there are differences then it’s as much an art as a science to try and predict how the differences will play out in “real life” for any particular person. In the case of component mattresses though … they often have the advantage of being able to rearrange or exchange layers so you can do some fine tuning after a purchase if your initial layer combinations don’t work out as well as you hoped.
Not that I’m aware of no (or at least none that are breathable because some mattresses do have completely impermeable or waterproof covers but these are usually more institutional or industrial mattresses or are made for children). You may have seen this already but there is more about mattress encasements in post #2 here.
Phoenix