Not sure if you misunderstood. I got confirmed that deleting any emails on Group Mailbox won't any member's mailbox items. Yes, did it that way, went well. Yeah, not an option for me now. Seems nice and faster action, than PowerShell commands Microsoft guys running.
1- It only deletes the individual message they received.
2- The only tool is to do a E-discovery search for email sent to this Group and then exporting that to a PST.
3- Yes, 50 GB is the maximum storage for an Office 365 Business Basic and Standard
You might want to setup an archiving addon for that mailbox or bump the license to E3 so they get 100 GB of Mailbox storage and then archiving is included as well.
4- YOu can setup a retention policy that deletes all items that is 2 years old and then apply to the mailbox and force the Retention Policy to apply then it will take a day or so but it will run faster.
https://docs.microsoft.com/en-us/powershell/module/exchange/start-managedfolderassistant?view=exchange-ps (To force the retention policy)
I wrote the answer 1 in a confusing way. The emails don't get deleted from each user mailbox that received it so no issue there.