Possible to find if Xbox was in insiders program from serial number or console code?

If you check your Microsoft Xbox account under Devices it should have your xbox details listed there.

Assuming the console isn't working, and you didn't change the HDD yourself, you will need to factory refresh the console. This is usually the only way to change Insider previews or revert to General release OS.

After the factory refresh, if the console is enrolled in a Insider preview, it will update the OS to the specific ring, Omega, Delta etc.

Check step 2 in System update solution for details on how to bring up the Xbox troubleshooting screen which has the Factory refresh options available.

Once that has occurred you can download the Insider app to the console and use the manage section to unenroll from the preview.

Check Xbox Insider FAQ.

If you installed the HDD yourself, I can't help you any further as self repair is contrary to the Xbox Terms of use.

I'm afraid that isn't possible as well.

Is it possible to find an account name from the serial number to contact them or get in touch?

If you look at system settings and updates it will tell you what build.You are on

Then check against the insiders os website to Sri if you are on public release or not

Hello Trevise, My name is Jesse. I'm an Independent Advisor and a member of the Microsoft community and I'd love to assist you today.

I'm afraid that's not possible to my knowledge. You'll need to sign in with the previous owner's account to check if the console is registered on the Insider programme. I don't think there's a way to figure that out using the serial number alone.