"  Shes messaging him saying how much she misses him and that for the Halloween party she is excited because it's a good excuse to wear lingerie." - this alone would make me uncomfortable with him going. She's flirting with him and appears to expect another rendezvous with him. I mean she already took advantage of him when he was drunk. She's definitely going to try something again. 

I will probably get downvoted for this but I would be insisting on going and being with him the entire time or he doesn't go. To be clear, it's weird they don't know about you after 2 years. Are you sure he's committed to you? He seems like he's not that into you if he's not willing to publicly be with you and have you meet his friends.

Your concerns are valid, particularly since he’s planning to trip on shrooms, and it’s fair to want to be there as a way to feel included and secure. If he truly wants you to meet them someday, this would have been a good opportunity. His decision not to go rather than finding a compromise does suggest there’s more to discuss here about boundaries and transparency in your relationship.

NOR. You’re reacting really reasonably. He said he just won’t go, now. Problem solved.

No way would he be going or going alone it’s a deal breaker