You're leading with two questions, here... and it kind of looks like an attempt to frame this as though I'm the one making assumptions. But I want to backtrack to how we got here, real quick:

Someone: Airbnb makes sense for a lot of people under one roof, like for a family reunion.

You: It's cheaper to rent an event space.

Yeah, sure, your commentary about sharing hotel rooms and amenities and group packages and blah blah might be relevant, but that's not what you said. You just said to get an event space. That doesn't have any information in it about where people are actually supposed to stay and sleep, which is what an Airbnb is for.

The comparison of Airbnb to Event Space doesn't make sense without all of that other stuff, at which point it all very much depends on all of these other factors like headcounts and group rates, so unless you're making a bunch of extra assumptions, you don't actually know which solution is cheaper.

Not usually. Last trip I took, which was to Seattle, the Homewood Suites extended stay was about 80 bucks less per day.