Well, you have many questions, some of which are fine (and you have possibly already a valid answer for them ) while some seemingly need to be de-composed and re-composed.
There is a bit of confusion between RAMdisk (like Imdisk that loads actually RAM volumes) and RAMdisk (like Winvblock, Firadisk or Gavotte's RAMdisks) that mount actually "disks" (whole disks).
But, yes, a RAMdisk is an image of a mass storage device (whole or partial) loaded into RAM.
What actually "boots" is not the "boot.wim" but the "boot.sdi", to which the wim contents are "mounted".
Sure you can have a "flat" PE actually this was used a lot of time ago for early VistaPE's, example:
The .wim format is NOT a volume image, it is a compression format (used for the contents of volume), much more similar to (say) .zip or .7z, it misses some (needed) parts for the mounting as a volume (typically the PBR and filesystem structure).
Most probably the WOF driver in Windows 8/8.1u1 is nothing but a "normal" "filedisk" (not necessarily a RAMdisk) that recreates on-the-fly the missing structures and exposes at the end a "normal" volume.
About the "traditional" use of .wim and of the wimmount drivers see (only seemingly unrelated):
The .sdi format on the other hand is a "plain" volume image with a prepended "header", see, JFYI:
The use of .sdi dates back to Server 2003 SP1, and Windows Embedded, though at the time it was used as a plain "volume image" and not as a "mounting container" for external files (still JFYI and to give you some info on the "history"):
And there has always been the possibility, using the above Server 2003 SP1 approach, to use compressed images, example:
The idea of having a "small core" and one or more "mounted" external images is also not really new, Sanbarrow at the time did an excellent work with MOA/LODR-packs: