Thanks for the feedback!
Stun Gas: My mistake, it's in Underdark Pg. 67.
Gehennan Morghuth Iron (A&EG): I overlooked this material! It's kind of weak even when it does hit, but like you said it's only 4K, and it would do double duty with other poison DC/effect boosters like assassination and whatnot. Definitely a good choice for a dedicated melee poison user.
Shapechange: I'm not really sure if poisons would stick around after changing or not, but I assume they would. It's certainly a cheap way to get a ton of high DC poisons (I'm looking at you, Balor!), though I do have to admit that I'd feel sheepish using Shapechange for such a mundane use as creating poisons
The Assassin Who Isn't An Assassin: I did the same thing in a game actually! I went Cloistered Cleric 5/Darkstalker of Bhaal 1, which along with my feats gave sneak attack, death attack, and poisons without being a rogue or assassin at all. It was quite fun, and the first module had a series of 10 spike traps or something laced with black lotus venom that I gleefully harvested. Campaign didn't last long enough for me to use them, sadly.