Hi srockrae,
Most people would use their wool topper on top of a mattress protector and then just use a fitted sheet on top of the wool topper. You can also use a mattress protector over the topper if you want more protection and use a fitted sheet over that but this can take away from the “feel” and performance of the wool topper.
As you mentioned you can also use a duvet cover to enclose a wool topper (it will be larger than the topper) but this isn’t necessary and I don’t know of any manufacturers that recommend or suggest it.
The care and cleaning instructions for wool toppers will vary between manufacturers and will depend on the specifics of the wool and the topper. There is more about care instructions for wool toppers in post #2 here.
Phoenix