Ooh, interesting situation, many thoughts:
1. They sent out 4 years extensions for everyone in the I-751 by now. Just read your posts, even with the extension your green card is completely expired in 2021.
So now even more reason for you to get working on your immigration issues! You need to fix this yourself since USCIS isn't. You can't just wait forever for an answer!
2. You have to be relentless to get an ADIT stamp. Call everyday, call your representative, make it your second job. Talk about your emergency, request a mailed admit stamp, say you have to have proof of status it is the law etc.
3. Definitely look into forcing them to give you an answer, 3 years for a response post interview is not normal. Unless your case has something very wrong with it, even a DIY writ of mandamus can get you both the removal of conditions and naturalization in a couple of months.