I am of the ones who don’t get that top sheet under duvet cover thing. Duvet covers are meant to be used on their own. If you get European guests, it’s very likely that they won’t notice/understand the top sheet and sleep between the duvet cover and the top sheet.
Just switch to a summer duvet or skip the duvet for warmer months 
The “smell test” somewhat puzzles me, I am not a neat freak but I am with @Lucy_R, each linen item that comes into contact with one’s skin should be washed between each guest…
Otherwise I would keep the same sheets for several guests, they still have that softener smell 
after a few nights.
Re. the ability to “see” if a guest has slept “full euro mode” 
after 42 stays and +150 guests, I have yet to see a stain from bodily fluids on my pure white linens.