I've been playing my way through this, noting all the secrets and how to get them, and...
...are you watching, Eater29? You've just reserved your own personal circle of Hell for the devious, devious positioning of one of the secrets in map 20. Better hope you've got enough ammo with you, because I'm sending in 100,000 revenants to deal with you for this! Without revealing too much, it's this one, which anyone else who has been tearing their hair out will identify in no time...
I've mostly been playing this WAD for the Keen hunt in map 28 - I signed up for this forum just to ask about those, and sure enough, it was exactly what I was looking for. I have a concern that the map isn't possible without defining a jump key, and somewhere on this thread or the "WAD club" thread there are instructions not to do that, but I see no way of getting over the Icon Of Sin pillars in the middle of the map without it, among many other examples. If the jump key is legit for this map, so be it.
So that sent me to map 36 - and for the first time on this entire WAD, I am completely stumped. There's a structure to the northwest of the map which I've found absolutely no legitimate way to get to, even though it's got monsters in it, and items to pick up. Whether or not I have a jump key defined, it's automatically disabled in this map, which might be deliberate.