Forgot to remove SIM card before sending back for trade in

You'll have to tell your carrier so they transfer the phone no. from the old SIM to the new you have now if you want to keep the same. Other than that, no further issue.

iOS does not store any personal data on the SIM, so no worries about your information being compromised. All the SIM has is the data necessary to get onto your carriers network for cellular service.

No, it's not going to be an issue. It's generally advisable to remove the sim, but not required.

Unless you stored your data into SIM but that shouldn’t be problem