I don't think you will have issues if you divorce and need to remove your conditions on your green card if you divorce. You seem to have solid proof, such as joint bank accounts, leases, health insurance, etc. I also do not think the postnuptial agreement is something you need to disclose. If they ask, of course you will be honest about it and explain what it is and why you did it, but if they don't, no need to say anything about it. And I doubt they will ask you about it, because how will they know?
Ultimately, a postnuptial agreement does not mean the marriage isn't bonafide imho.