I have a mem foam mattress, that is basically just a huge slab of memory foam - no fabric etc.
so i realised that it'd be hard to clean, & i was worried if i spilt drink on it, or sweated heavily, or even had someone wee in it (my ex used to wet the bed when he drank booze)
so i looked into matress protectors but in the meantime i put about 3 bottom sheets on it & also my old mattress topper - which is a 1 inch sheet of memory foam in a fabric cover & in a duvet cover - i figured its better if anything was soaked up by those then the main one
however i've found that all those thing sort of defeat the benefits of memory foam - there is now not much 'give' in the surface, & the thin mem foam sheet has somehow bunched into a ridge - i assume my weight shifted it but all the sheets etc have stopped it going back to flat shape.
also noticed my body dent seems to stay there a very long time im on 9st 5lb & only had it about 6 months so it must be all the layers of fabric - which is about 7 layers
I'm going to try putting a satin sheet on the mattress first before the other sheets so hopefully they wont bunch any more & will slide back as soon as i get up - mem foam has a weird texture so i can see why things would not move freely against it.
but i still need a 1 sheet mattress topper/protector that moves with the mattress, but isnt plasticky & noisy
All the worries you have about soiling the memory mattress come with any mattress. Like you have already said, the idea of a proper mattress cover is to take the bulk of any spill along with one bottom sheet. Anything else piled on top defeats the memory foam. Sweating in bed is normal especially in summer but I think that's why we wash / change the sheets.