Yes, I agree with @Sumit-Kumar Marketing Channels are processed at the time the data is received... also, that Data Repair is costly.
The other options are Classifications (which can go back 6 months)... but this is going to be limited, because this will only look at values collected in one dimension, and not be fully considerate of the wide processing needs....
You could also try to create Segments, but again, this will be imperfect... but might be able to be used for some basic understanding.
However, the most likely scenario is that you fix the processing now and move forward, and accept that your past data is corrupted / wrong.
We've all been there... it hurts... but take it as a lesson learned, and a teaching moment for new hires of what can happen.
Good Luck!