korell: Then, on playing through the first area after the Hub (The Crypts I think it was called) I ended up in a situation where the game wouldn't let me open a door until I had picked up all of the weapons, but the area I was in required dropping into a pit that you can't get back out of, so having missed a single weapon I was then stuck unable to progress. Not a great impression, and hence not recommended.
korell: Turns out you are supposed to be able to jump and crouch, as well as swim underwater, in Eternal Doom IV. Jumping may well have got me out of that pit in order to backtrack to find the missed weapon.
Sounds like you're making some progress. I'm playing through HacX right now. It's been a pretty enjoyable mod so far. Parts of it remind me a bit of Deus Ex.