Bookmakers only close their gates on live bets. I frequently bet on sports events, especially soccer and I have never seen bookmaker closing their gates on any event except live bets.
Btw has any casino done any kind of poll about who is going to win? I'm seeing polls of BBC and other media, they all favour Kamala Harris. This betting moment is a huge moment for every casino, they are lowering odds for Trump but increasing for Kamala. It will be cool if any casino hire own, independent team and makes a poll in US states. It will cost a lot but I think that marketing strategy can be built around it and the casino can financially profit.
As far as I know, bookmakers calculate odds factoring in two components:
1. What people bet. If someone places a big bet on one side it lowers the return you get for that bet and raises the return on the other side.
2. Statistics they handle.
I don't put my hand in the fire that everyone does it this way but at least that's the way I understand it is.
By the way, does anyone know until when it will be possible to continue betting for the US election in MW? I imagine until the polling stations close, no?
I also thought as you said, bookmakers try to balance odds outcome in optimal way, e.g. if someone bets big on one side, they'll lower this side and increase odds of the other side but then I have this question - How is that odds are almost the same everywhere? It's almost impossible to do arbitrage betting. For making bets, I mostly use local bookmakers and I have seen the same odds everywhere, on each local bookmaker.
Once I also saw a job posted where a local company was looking for a person who was actively watching sports and also had a degree in statistics or data analytics. I think that there is a team full of mathematicians and data analysts that work on making odds and then these bookmakers get odds from these odd providers. It's similar to slots, there is a group like NetEnt that works on slots and casinos hire their products.