If you don’t control ten or more snow permanents as your upkeep begins, the last ability won’t trigger. No player may take any actions during a turn before the turn’s upkeep begins.

If Marit Lage’s Slumber leaves the battlefield before its last ability resolves, you won’t be able to sacrifice it, so you won’t create Marit Lage.

If you don’t control ten snow permanents as the last ability resolves, you can’t sacrifice Marit Lage’s Slumber.