Regarding your questions:

Q1: What does the ENVELOPE_ALLOWANCE_EXCEEDED error mean in Docusign sandbox?

It means you have reached the maximum number of envelopes allowed in your developer sandbox account and cannot send more.

Q2: Can I reset or increase the envelope allowance on my developer sandbox account?

No. Docusign does not reset or increase the envelope limit for sandbox accounts.

Q3: What should I do if I reach the envelope limit in my sandbox?

Request or create a new developer sandbox account to continue testing.

Q4: Can I use a trial (production) account to create new Integration Keys and continue testing?

No. Trial (production) accounts do not allow creating new Integration Keys. Integration Keys can only be created in sandbox accounts.

Q5: What is the proper workflow for testing Docusign integrations?

Create Integration Keys and test all features in your sandbox. When testing is complete, submit a Go-Live request to move your application to production.