1)From what I understand if you qualify for the sit-n-learns you will be automatically entered into that months freeroll. If you don't show up to it they just blind you out. It sounds like the tournaments that ran this month had less than half of the players in them, because no one knew exactly what times they were as there was no real announcement or no e-mail reminder or anything for players. (Use ctrl + R to find out what tournaments you are registered for while in the FT lobby)
2)Not sure, but if i understand your questing as being "Is there a limit to the number of sit-n-learn freeroll tournaments you can play in a month, then I believe the answer is "No"...there is no limit. Again this is just my understanding, you might be better off asking this question on the FT forums.
3)A FT moderator posted in the forums on monday (2 days ago) that he hopes to have preliminary pricing out by the end of the week, and then open the store up in 2-3 weeks. Hopefully he is right, but then again on March 10th he made a post saying the store would hopefully be open soon and well...that was a month ago.
4)Yup you can switch challenges. I have 4 challenges up right now where i have 2-4 things completed on each one. I've found that for me the pre-flop challenge is the only one I can complete without making an effort.
5)As of now, no. The only way to get points is to complete the challenges. Each one can be done 10 times and there are...I think 6? challenges right now. It was stated that sometime within the next week or so they should be releasing 1 or 2 more challenges, and it has also been stated that they will try to release new challenges every few weeks.
If anyone has anything else please feel free to ask...I've been studying up on these challenges quite a bit and hope that the store offers some good deals! If the store is at least halfway reasonable this should be something good for everyone. I mean...you basically get stuff for not doing anything out of the ordinary.