About the mushroom event

I need to ask a few questions :

First how do you complete the challenges other than picking 100 shrooms?

Second what do you guys think is the most "worth it" item/s to buy with the points?

Third anywhere you can farm them efficiently?

Fourth i heard there is a "mushroom questline" ? How do you get it/trigger it?

I just finished the story yesterday and joined the North and boy am i having a rough time