With buying this game, you get the expansion packs. What are the names of them, from what I found out with the base game and the expansion pack levels, there should be 6 Episodes, but I only see 5. Do I need to beat the first level or chapter to unlock it?

to play cryptic passage you gotta activate it in options -> manage add-ons

Not entirely related, but do you know how to revert to the base game once I've loaded the add-on? It seems that if I change my mind about which episode to play I have to close the game and restarting but there should be a better way...

I'm pretty sure that at the moment this is what you have to do. Once Cryptic Passage is enabled, the Manage Addons option is still present but you cannot choose to disable it, only to select it again which will just reload Cryptic Passage. So quitting the game and reloading is then required to get back to the original Blood episodes. Maybe the devs will address this in a future update?

I see. Well, it's a minor nuisance anyway but it was still a bit annoying as I was trying out the "Load addon" feature (I also tried to load Death Wish). It shouldn't be a problem again, but yeah it would be nice to just be able to revert from the menu.

To answer the original question. The expansions were Cryptic Passage and Plasma Pak. As already stated Cryptic Passage is available in the addons section while Post Mortem is the Plasma Pak episode.

Cryptic Passage was developed by a separate team (Sunstorm, who also made two of the three expansions for Duke Nukem 3D) so is not really considered part of the same continuity as the others. Whether you play it or Post Mortem doesn't really matter but CP is considered the fifth episode by many because 1) it came out before Plasma Pak 2) internally Post Mortem is regarded as the sixth episode