These are good questions! Gringotts is still reasonably new, so we're still working out the kinks.
1. I'm going to say that classes and ECs count, as long as you have at least three posts in each thread. Also yes, subforums count.
2. School-wide events were originally things like the Inter-School Games. We don't necessarily have them every term, although House events that are open to the whole school would certainly count!
3. I'm gonna go ahead and say yes, they both count.