I am only guessing here, but I don't think they'd freeze your account immediately. I imagine they would send you another reminder and give you a little extra time to bring the account status back up to fully paid-up.
As I said though, this is only a guess, the best way to get a definitive answer would be to ask DreamHost support.
That is an interesting question and, once again, a question best asked of DreamHost support.
I would think that once your hosting account has lapsed you would lose any accrued disk-space/bandwidth bonuses and would have to start from scratch. The domain name would of-course remain yours, as long as you don't let the domain registration lapse, this is true regardless of whether or not you maintain your DreamHost hosting plan.
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