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Close some of indices that are not necessary will take care of line limits futures are only thing live after 4pm

You are missing one major point (sorry for not mentioning it earlier): VIX options are based on futures, SPX options are based on SPX index. So VIX front month options can trade higher than back month when VIX is in backwardation. For SPX, this is irrelevant - options are based on the same index, so front options cannot be more expensive than back options. Not theoretically and not practically. It works exactly the same as options on stocks. This is why there is no margin requirement when you buy SPX or RUT calendars - you cannot lose more than the debit paid. And if you don't close it on expiration day, the broker always has an option to liquidate it (as a spread or one by one).