The cache is at the posted coordinates, but you cannot log a find until you have fully completed the challenge!
All cache types count (but not the new challenges or waymarks).
Create a public bookmark called "Finds by Placed Date Challenge - your caching name" these caches that you found on each of every single calender day. Retain this public Bookmark for 3 months after finding the final. You can also Run GSAK and post a picture of your Finds by Placed Date.
Once you sign the physical logsheet and complete your list, you may log your find.