Most of the anti-bedbug sheets talked about on this forum are not zippered mattress covers. Rather they are simple fitted sheets (sprayed with permethrin) that fit on the top and sides of the mattress. The idea is that the permethrin treatment will deter bedbugs from walking across that sheet to your body, and will instead pick on someone else. There is not much scientific evidence that this will be effective, but it is likely worth trying. Many albergues already have plastic covers on their mattresses. I can't imagine carrying the bulk and weight of a zippered mattress cover and wrestling it on and off the bed every day, especially since bedbugs could be in the other mattresses in the room, as well as in furniture and crevices in the floors and walls, etc. And you pointed out how difficult it would still be to guarantee no hitchhikers in that mattress cover.
Unfortunately there is no sure way to avoid bedbugs on the camino. You can reduce your risks - first of all by inspecting the bed and area and leaving if you see signs of bedbugs. Then, use a treated sheet under your treated sleeping bag, and enclose your treated backpack in an airtight bag while in the albergue. I intend to take a second airtight bag to enclose my sleeping bag and night things during the day inside my pack. That way, if I develop itchy spots during the day (from being bitten the night before) I have isolated the things that have been exposed to bedbugs. The itchy spots can take a day or more to emerge, and some people do not react to the bites at all.
The most important thing is to avoid bringing bedbugs home. You need to figure out how to get all of your stuff treated upon arrival at home, without spreading them around your house. Enough heat or cold will kill bedbugs. You need to heat them to about 55 C (130 F) for 20 minutes, and if you wash them first you will need to get them dry first and then add 20 more minutes. Or you can put them into a freezer at -15 C (0 F) for a week or two. Some but not all regular fridge-freezer compartments get down to this temperature. Finally, you'll just have to inspect and wipe down things like your cell phone.
There are links to a lot of information from reputable sources of information on the Camino Resources page at the top of this forum page.