I bought 2 of everything. Every time a guest checks out it will be replaced. Saves on my time and effort (and electricity cost) too since I can then wash in the off-hours and then air-dry on the line the next morning. The quilt and coverlet are to be washed every evening after ever visitor in the evening, the sheets are washed in the morning after I remove them. Why this way? The sheets are more likely to take longer as I use the "sanitze cycle" on them (1hr 35 min) and then they can dry in the afternoon sun. The quilt and coverlet can use the "bedding cycle" that (afternoon or evening) and dry the next morning before I need to put them back on the bed.
As for your question re: the flat sheet. I don't know! Speaking as a guest at hotels; I've been taught that one uses the flat sheet all the time over you to protect from "unclean bedspreads" (ie: sleeping between sheets protects you from critters and germs in the bedspreads that are almost never washed between guests at cheaper hotels. BUT some people think 2 layers of sheets protect from critters in the mattresses...
You can't please everyone. Honestly? Don't worry about the wear and tear on the duvet or quilt. My answer? You are gonna think I am crazy, but my real answer? Invest in a larger washer. It is worth it's weight in gold. I got a 4.5 cu/ft and it can wash sheets, quilts, coverlet and shams in a single "bedding cycle" (if I wanted to wash in one cycle) ... and it's amazing on power. We've saved 1/4 the cost of the washer on power alone - in less than a YEAR! (We got the WM5000HVA in white WITH the sidekick) This configuration allows us to watch the kitchen towels and smaller things when using the larger washer.