TLDR since I ramble. I don't see anything wrong with our version of detente and having enemies at the same parties, espically open events. I don't think anyone is getting a "free ride" OOC, or rather everyone is. I'm (IC) anti-slavery and anti-pirate, but I've gone to The Last Gate and I know other people who are anti-pirate have as well. I've seen Gallente and Minmatar in Amarrian and Caldari hangouts, even during the war, and the opposite of course. So, if someone has a problem, just make it IC and sit back and watch the fireworks. Seriphyn is no different from the hard liners in every other faction who shout death to <INSERT NAME HERE>. The rest of us shrug and nod, and get back to pumping our dates for information. Reasons below.
I don't think I've seen anyone in this topic make this point yet, so I'll spit it out. I think people are looking at this too much like we would in real life. If we go to a party, we'd be upset if <insert RL enemy group here> also showed up, espically if they had tried to kill us before. But EvE isn't real life, and our characters are not who we are.
When I started getting into RP, ICly I did stick to people mostly in my faction/allignment because why would I pall around with my enemy? Then ICly, a fellow pilot (Takitoo if anyone else knows her), pointed out that because we were capsullers the rules are different for us. We travel around, never dying, never being in a place that wouldn't kill us if we tried anything, and we don't have any real need to fear being with our enemies. CONCORD or something similar is always there watching us and everyone else, so we smile and nod at our enemies, maybe even dance and wine and dine them. She said it better (this was a few years ago), but hopefully I get to point across.
Or think of it like this. Its also like the cold war with the US and Russia. the US didn't nuke Russia because Russia would nuke the US right back. I don't walk into PIE headquarters and set off a bomb on their clones, or hire someone to do it becuase (IC) that would be the end of it. PIE would find out what happened, and do the same to me, and maybe EM too, just because. And EM hits back and includes a few more, and so on and we all die. So we fight our wars through the "proxies" of our ships and clones, and like the US and Russia we trade insults and seduce each other and go to the same parties to spy or maybe just to have a good time.