That's correct - though you still need to log in to buy a Streak Freeze after it's used up, so you can't skip more than 48 hours. You can't "buy" multiple Streak Freezes in advance.
The Weekend Amulet is only offered after completing the lesson on Friday, as far as I can tell - you can't just buy them in the store, and they expire after the weekend is over whether you used it or not.
I got a bit over a 500 streak but stopped after that.
I felt that spaced-repetition on the words was more effective than whatever DuoLingo was doing, and DuoLingo's lesson for the language I was learning was not well maintained.
The gamification techniques DuoLingo's used made it much easier to maintain the streak. Albeit, it was really easy to just do DuoLingo "to get it done" rather than focusing on it as part of study.
The Weekend Amulet is only offered after completing the lesson on Friday, as far as I can tell - you can't just buy them in the store, and they expire after the weekend is over whether you used it or not.
(I'm currently on an 872 day streak.)