I actually think I only had 3 stretch challenges as well but when I bought the witches outfit, it automatically completely 2 other challenge I didn’t see before (I thought I just didn’t see them). Maybe with every stretch challenge you complete, they replace it with a new one?
You don’t always need to get all of the free earnables to continue on to the next page (I.e. the first page you need to get all 4 but you only need to get 3 more from the page 2 to access the 3rd page) it tells you how many items you need at the bottom
Since it’s from this party weekend in May 2021, I would think it’s available for diamonds by now. I think I got it in a tb sale so you could wait for the next one and see if they’ll add it to the sale again