With prefolds is there any other folding other in folding the two outer lengthwise inwards that needs to be done to put the nappy in the cover? Do you use clasps (not sure what the technical name is) or does it just get shoved in so to speak?
I am not very good at explaining this in type but... I always folded mine in trifold but leaving wings at the top to wrap around babies waist, it looks like an upside down triangle, you fasten using snappi's although if your cover is snug enough you don't necessary need to.

Also I just realised that covers don't have to be PULs. Is that right? You can use fleece/minkee/wool as appropriate covers?
I always used fleece/minkee with mine, yes you can also use wool.

I think it does depend on the baby and the prefold, as I said above I used Maz's and they were great, I could go up to 3 hours between changes.