Geez, I don't know that I can offer help here but I will say that the advice you've got so far seem really good to me.

Most importantly, you didn't screw up. You are looking at things from both sides and finding understanding for both. I am sure you thought she would see things a little more rationally but we usually don't know it until after the fact.

In my separation and divorce my mom was mad to but she was mad at me. She had a hard time because it was my choice. She adored my ex. (he was very good to her) She also believes strongly in marriage vows as do I. I simply explained that this is what I thought was best for me and it was my decision. Eventually...after some time....she got over it.

I don't know if anything I said helps. I wish you the best in however you decide to handle things. I have every faith in you that you will find a fair, diplomatic and caring way to straighten things out.

It is very impressive that you forsee it eating away at your relationship and you are willing to confront things so that doesn't happen. Your step mom is very lucky to have you in her life.
Hugs,
Heidu

The highest reward for a person's toil is not what they get for it, but what they become by it.
John Ruskin