Well ....
My professional opinion would be to contact the local inspector and arrange a meeting to go over what is needed to make it acceptable to the AHJ.
This way your backside is covered when the new owner starts to discover what ever else has been hacked on the property. This also keeps you out of hot water with the the local AHJ's office. Keep in mind once you touch it - it's yours No mater how "quietly" you walk away. Better to spend some money and do it right , then to have it come back and bite you in the azz later!
I would most definitely make the grounding and bonding correct regardless.
As far as back feeding a breaker in place of the factory main... Well the code really does not tell you that it can't be done, it only tells us how to do it legally. My thoughts on removing a factory installed main is that you would be altering the UL listing on that panelboard by doing so.
