1) You are correct
2) IP address changes are rare, so you may end up with the same IP address for a year or three. But if something blows up (server or network configuration), they may have to rush you to another IP address.
3) For a planned change, I expect they'd give you a week or so advanced notice, but I'm not a static IP address owner, so I've not experienced this. But for an unplanned change (see above), you won't receive notice until after the fact.
4) Downtime would be determined by the Time To Live setting (length of time between DNS refreshes). When a change is planned, they'll crank down your TTL to a matter of minutes to minimize interruption.