First, I don't have any info on if the Friend League is permanent or not. I could go either way - I usually cache alone (or just with my family), but it is kind of nice to see where my caching friends are caching and what they've done.
But in regards to the challenge aspect of your question:
1 - I don't know that it would be possible to create a checker to document your Friend League points. It's not really one of your "stats". But perhaps the techies out there would know how to do that.
2 - If the challenge were based on the total points on the Friend League for a given week, the caching actions of your friends would have more to do with you qualifying for the challenge than you would. (e.g. Your friends go out caching and rack up 100 points. You don't go caching at all that week. Do you still qualify?) But then if the challenge were put back onto your own caching activity for the week, it's no longer related to the Friend League.
3 - I've not seen a "cooperative"-type challenge that requires you AND other cachers to do something. The only thing close is The Collaboration Challenges (http://coord.info/GC2KAQ0
by HikingSeal for example). Those require you to go caching (not events) with at least a given number of different cachers. But those are all grandfathered in as old challenge caches. So you could pass it by your local reviewer and see what the guidelines are for cooperative challenges.