I'm in a similar situation for Spartan Strike. Although I am at 88/90 objectives complete. I didn't want to get to 90/90 yet until an answer about what to do comes though.
I've gotten a response by message from 343 that they will take a look at it - I'll update you when I get a response.
I had referenced your situation to them as well.
I managed to get the achievement on both platforms with a method described in this link.
I understand this won't work for you, and have asked the team I spoke with to try to investigate for people in your situation.