Hi Steven, to answer a few of your questions...
"It appears as though it is essentially a drybag that is placed into a mesh pocket and then held in place by the straps. Correct?"
Not really, without the dry bag it is a pack in its own right (see below) so you could take the orange dry bag out and you will still have a pack body (see pics).
"If so, can I assume that one could store wet gear between the bag and the mesh? It looks like you have something stored in that manner in the 9th photo from the top, but just want to verify"
I just had a check and indeed you could store something between the pack and the mesh, it may compromise the comfort or breathability of the matrix mesh but I am not sure. You could also put cool water behind the mesh to cool you down, but I have not tried this.
"Also, the balance pockets appear to just have the small drybags inside a larger pouch. Does any of the drybag material show through like it does on the main bag or is it fully enclosed. basically, do I have to use the drybags on the balance pockets? and are there little mesh pockets on the front of the balance pockets."
Indeed that is the case, the front pockets are sturdy little bags with dry bags inside and yes they are mesh pockets on the front (see picture), I find these pretty handy and have a little clip so you can attach a camera without worrying about it going missing.
front dry bags
front bags from top
Main dry bag
Main dry bag 2
inside of main bag without dri bag - you can also see one of the removable stays at the bottom of the pack. This pack has heaps of removable aluminium stays scattered throughout the pack. I am not about the mechanics of these and how they operate, but it looks like there has been alot of thought put into them.
Also, thanks for the condolences Roger, it still ended up being a fun trip and I would have liked to have test out the MM55's waterproofness but my hiking partners did not (believe it or not)