I was expecting to see a 3rd expansion called "Plasma Pack", but I only see a Cryptic Passage. Was I misled? What is the Plasma Pack?

The Plasma Pack is the episode "Post Mortem" within the main game. The original game had only four episodes, ending at "Dead Reckoning."

Ah, ha. Thanks! I didn't see that in any of the docs or .TXTs... probably just missed it.

Regardless of he naming of the maps, Cryptic Pasage is supposed to be Episode 5, since it is the only other official expansion pack and it came out before Post Mortem. (But you can play them in any order, since they have completely different storylines.)

I think you'll enyjoy them both: Post Mortem is the one with new content (new enemies, new boss etc.), while CP uses only the old content but has a far more amazing level design.

Ah. In that case I am inadvertently playing them in the correct sequence :)

It seems rather odd that Cryptic Passage wasn't worked into the main menu and executable as well. That would have been nice for the OUWB release, at least.

I would guess that CP must be more customised than PM based on this separation, but it sounds like if anything it's the other way around. Oh well... I guess there must have been some reason for doing it that way.

Probably the reason while CP isn't incorporated in the game that it wasn't made by Monolith but a different company (Sunstorm Interactive) and is only approved by the makers as an official add-on.

Still, I think they did a good job, I actually like CP better than the "real" add-on Post Mortem.

There is, it's called Bloodbath. All the dedicated deathmatch/capture the flag maps fill up the episode 5 slot.
In game, hit T and type "mario 5 1", and you will can wander through the maps solo.