I beat episodes 1-4 and now episode 5 (post mortem) says E6M1 on the map screen... is it E5 or E6?

E5 is Cryptic Passage, if I recall things right, Post Morten would be E6 in that case. Yeah, with Cryptic Passage being a separate executable it's a little confusing.

Actually, they are the bloodbath maps.

If you level warp (mario) to E5M1, that's where they start.

Cryptic Passage is a third party add-on that got bundled with One Unit Whole Blood.