1. The 645 does not rotate however there was a 645 vertical back which however only gave you 12 shots, so it was pretty rubbish. There is a 645 mask for the finder but it is very tricky to find one. I though often of getting a 645 back for my blad but in the end I just crop, yes it wastes film but I still have the option of 6x6.
2. By coverage what do you mean? I have a PM90 and you see the full frame. I use it with an acute matte D and it is nice and bright, I prefer it to the WLF. I do like the WLF on the tripod but once you start getting into acute angles it is inconvenient.
Also, if you plan on using long lenses the 500CM mirror cannot show the whole frame. I don't have personal experience as both my bodies had the GMS but others can chip in and say how much this is an issue or not.