YC is arguably the best accelerator in the world. Maybe you don't need an accelerator at all, for those who are interested, applying to YC is definitively worth it.
Do you need to be "accelerated" when you already have all of the above? It could be a lazy way to move forward. Which costs you %7 of the company. I believe you can do it yourself if you made it that far already.
Yes. "Some active users" needs to be millions (unless you are b2b)
We have access to 400k candidates, a real product, and real clients (Pizza Hut, 7-Eleven, just to name some) and we got rejected.
Edit: and I'm OK with it, we just need to work harder. We do not depend of YC to be successful, but I see all the advantages of being part of the YC club.