First off, I have to give you major props for such a detailed and specific question. I used the links you provided to better understand the problem-space as I never use that functionality myself and it was click, click, up to speed.
I was happy to take my time to test out a theory on your behalf since you were so respectful of my time. Thank you!
I tested out an approach that treats your +email address as an alias. Seems to address your needs. The only caveat is that you need to prepare any + addresses in advance, or on-demand.
+ Address as a Gmail Alias
- Navigate to the accounts tab in Gmail settings (https://mail.google.com/mail/u/0/#settings/accounts)
- Under 'Send mail as:' click 'Add another email address'
- In the email address field type in your +email address
- Leave 'Treat as an alias' checked, optionally customize the Name.
- Click 'Next Step'
- You are immediately returned to the settings page as the normal authorization phase to confirm ownership is redundant.
- Make sure 'Reply from the same address the message was sent to' is checked under 'When replying to a message:'
When replying it might appear that Gmail is reverting to your account email however do not despair, this is not the case and you can confirm that by running some tests using a second email account (Gmail or otherwise). Gmail is just showing off the fact that it recognizes the email address is essentially one and the same.