1) I think this depends on the rate limits that the endpoint you're calling has imposed. If you send 50,000 requests at once to an endpoint that has a 5,000/minute limit, of course you'll run into issues.
2) The rate limits vary per endpoint. The number of calls was reduced for some endpoints and set for others last June (2022) and the documentation should be correct.
3) Currents. It's a (paid) Braze feature that automates the export of message engagement and behavioral events from Braze to a data warehouse or an analytics platform. For imports, I'd advise you to contact the support team and ask if they can run a bulk import, if it's a one-off.
