Threat is a double-edge sword. You can also be banned from a campus and then you would indeed have to put that lawsuit threat to the test. The existing case law is not exactly overwhelmingly in a parent's favor. Threatening a law suit then actually financing one through a very possible appeal or two, not to mention the time commitment, typically drains families. Schools usually have bigger coffers than families, and they now have a unified process for repelling "federal intrusions" in the form of OCR investigations.
The NSBA is also launching a renewed effort to fight back against Tustin. I can't agree with your comment on liability. They have liability regardless of what you sign. Each state may have its own statutes that release identified personnel that are trained to administer epinephrine, and it may further state that self-carry is contingent upon child's doctor attesting that child can self-administer.
They are not released from in loco parentis, duty of care, ADA or 504, etc. However, they also still have government immunity and qualified immunity, qualified immunity being nearly a "freebie" as long as whatever was done was done in good faith.
I'm not downplaying your role as a parent advocate, but I would say that it's unknown what combination ultimately led to the outcome in the 504. The idea of threats via email is not something that would get results from many districts. It may end in a ban from campus or worse.
Advocating under ADA is a protected activity -- within limits. On that note:
Retaliation claims and protected activities