No Nostr stats are accurate, but Nostr band takes a large enough sample to be a good measure.

Yes. You have to query every single relay to get accurate stats, which is not even possible, since you cannot query every private relay. However, nostr.band is close enough to be representative

It pulls from the largest relays. Gives you a good read of activity on the major English speaking public relays. That's about the best you can get.