The easiest way to test in Sandbox, once registered, is to use the CreateIdentity API to create a new identity. If your sandbox consumer is registered and setup correctly, you should see an identity.approved event. Reach out to Support if you do not see the message.
If your consumer failed to process a message, Paxos will attempt to retry for up to 24 hours, read the full retry policy for more information. After this you can use the List Events API to fetch undelivered events and process them.
Yes, webhook invocations to your consumer endpoint are rate-limited to 10 per second by default. If you need higher throughput, please contact Support to discuss increasing your rate limits. For more information, see our rate limits documentation.