Does anyone else think that Mac may make an appearance
Someone has to cater it. Mac is kind of the perfect person to bring in on that; he's known to be neutral enough that he won't poison the entire conference. He wouldn't really make sense to me as an arbitrator, but I could see everyone trusting him enough to handle their food; every person who has entered Mac's seems to know him, and has given him some sign of respect—even Maeve.
But has Marcone ever set foot in Mac's? I don't know that we've seen that on page. I'm sure he knows about it. Would he think to use him? Or would that be Harry saying "Damn it, people have to eat. Believe me, you don't want a bunch of hungry predators in a closed room with only humans to eat. I've cleared Mac for security already, and we can trust him."
Imagine a scene where most or all of the delegates are meeting downstairs in the convention area, and while Harry is doing whatever his security work involves; let's say he's facing off against Fomor security, and he receives a message from Lara Raith that says something like this, "I need to see you in my room after this session ends. There is a plot that concerns us both of us that you need to know about and I need to speak to the Winter Knight. I need to know what Winter's position is on (fill in the blank)." (Of course Lara would throw some sexual innuendo in any note she wrote to Harry.) So if Harry decides to take Lara up on her offer he has to go from one type of dangerous situation and environment into a different type of dangerous situation and environment and yet it's all contained in the same facility.
I agree. Splattercon!!!!'s location is a good fit. Though didn't Harry say something about bad juju hanging around places like that permanently? But he's also frequently commented on how often the same locations seem to crop up (like the Full Moon Garage), so that'd be a perfect segue into revealing where it'll be held, and can explain Harry's heightening anxiety beforehand.
Hotels are awesome locations for stories, because it's insanely convenient to have everyone in your story all in the same place, but separated. With magic involved, you might even have each occupant's room changing dramatically from one to the next. Also, it'll give Harry too many freaking stairs to climb, and you just know he'll be complaining about it the whole time.