@Lemontree -- I agree with the idea of requesting seats at the farthest back table and close to an exit. I wouldn't explain any further than that. If she pushes, say you might be slightly late (or that you might have to leave early) and don't want to disturb the event. If she really pushes, I dunno...just keep saying that it is important. Personally, I don't think it's worth saying anything about the crowd or PTSD issue.
Awhile ago I was in a relationship with someone who hated going to gatherings and he either ducked out early or cancelled at the last minute (meaning I ducked out early or didn't go to things, since I was a stupid girl who did what my man wanted to do instead of what I wanted to do, but that's a different story). All I'm saying is that this is common in a lot of people, not just people with PTSD.
Knowing you prefer the back might make things easier for her, too, since she might not know exactly where to seat you (in front with family or farther back with acquaintances).
...We went to see American Sniper the other day. We went to a theater in an unpopulated nearby town and sat mid-theater close to the exit and away from people. It was a bit of a game we both played without even talking about it. If someone sat close, we'd look at each other and move to another row. We probably looked like a couple of crazies, but whatever. I said "This chair is broken!" loudly during one of the moves. Neither of us knew what kind of reaction he'd have. Thankfully, everything went smoothly and we talked about the movie at length afterwards. (Nightmares have started getting worse, though I expected that, and he's going camping this weekend to process stuff and be on his own for a few days.) Has anyone else seen the movie? How did the viewing experience go? Did it trigger anything?