Hi flybear,
There’s no especially magic formula to knowing what kind of topper your grandma would prefer except to have her test various ones for PPP. Likewise, talalay vs Dunlop is really a personal preference thing.
That said, in my experience most of the retailers of customizable softness foam toppers (e.g. ild) will go by her weight if they have nothing else. I would phone them and ask their advise. For blended talalay, I’d guess you are advised 19 or 24 ild, but that’s only a guess and you should solicit the retailers advice.
Those toppers that have a mixture of materials (usually latex and wool), or more elaborate design / quilting in the cover, tend to be non-configurable (or sometimes have a couple of differing thickness).
If you get a bare latex / foam topper, then yes a cover is a good idea. Latex breaks down less quickly when properly protected. Also, a good cover should compliment the feel you’re going for, and not interfere with it (not unlike a mattress protector). A duvet cover is the wrong dimensions, so would be less preferable.
Any of the members here have Phoenix’s stamp of approval. There are others also that sell quality products. The Clean Bedroom in the USA has some options beyond simple latex layers if you want to see some examples like that.
For simple latex toppers, there’s a list in the DIY thread:
https://forum.mattressunderground.com/t/foam-factory-any-good