Just to clarify: Unattach does not corrupt or mangle emails. Because Gmail does not expose an API for modifying the original email in place, Unattach creates a new version of the email with attachments removed/downsized while preserving the email's content and metadata.
If you need the original email for legal/evidence purposes, you can back it up first, either through Gmail or Unattach.
Yes, that was covered in the other thread. Presenting a backup as evidence has a lot less strength than the original.
If the other person doesn't have the email (they deleted it) and you don't have the original because Unattach deleted it to replace it with a different copy then we're only left with an offline backup on 1 person's machine. Who's to say that offline backup wasn't tampered with (modifying headers, etc.)?
You're trying to establish a chain of custody here. The only person who remotely cares about the forensic integrity of the message is the judge (and the jury if it's a jury trial) and it seems like an argument you would like to make is "this email came from Google's servers so we didn't tamper with it" - but the judge won't take your word for it, you will need Google to substantiate that and they won't unless it's a workspace account.
If it's a workspace account, then your org already has a retention policy.
So again, what problem are you trying to solve and in which jurisdiction?
I think the issue with localizing it to a specific state is oftentimes contracts or working agreements have jurisdiction locations that align with the company. In other words, it might not be in NY. It could be anywhere, including different countries.