Thanks for the feedback, it is very appreciated. This issue is an older one that came out in April of 2016 so Bitter Pill would not have been available. I believe my original thought process of the design was to keep the list relatively short and focused on poisonous effects and aberrant bodily changes that could be considered a mutation. The concept behind this class was the Toxic Avenger comic/movies I remembered from my younger years.
As the discoveries were part of the pretty good sized list of toxic powers it was intentional to keep the list small so it would give a character the ability to pull options from the Alchemist class directly but I didn't want it overloaded so that many of the other cool toxic powers would be ignored.The character only gets 9 total (unless the Extra Toxic Power feat is taken once or more to add more) so by limiting it to 8 discoveries, it kind of forces the player to choose at least one other toxic power.
However, absence from the list does not mean a discovery...See more that may fit well within a game couldn't be hand waved in by the GM if desired and as any 3pp supplement this class and its abilities is a guideline than a concrete rule. If playing more of a wilderness based game I could see why the Feral Mutation or Bitter Pill discoveries being viable options or even possibly constructing an archetype with them added to the list.
I am considering doing a compilation of hybrid classes in its own book to update things from when they were originally created, especially now that Pathfinder 2.0 is on the horizon and there won't be much more official Paizo material coming out.

Really appreciate the response TKW! I hadn't realized the release date was 2016. :P I know that I will be adding at least Preserve Organs to the list as it certainly makes sense, more so in light of what you mentioned (aka The Toxic Avenger, which btw is an awesome movie). I imagine it as his organs actually moving around and/or migrating throughout his body. Good stuff indeed.

Cheers