1. It doesn't matter when you did the centum, just that you did it
2. You must sign the log in the physical cache at the posted coordinates
3. If you want to form a group of 100 people and each do one cache, go for it. Good luck finding 99 other cachers willing to do that. If you can sleep at night having done so, then by all means have a blast.
This is a game! Play how you want, log how you want, hunt how you want. I cache the way I cache, it happens to be the way that Shuman Road Searchers, Dubord207, mmface, lena484, mapachi, et al cache, but if you want to log a find that you didn't find, log it, you're only cheating yourself.