I am glad you came to a solution that you are happy with, OP.
I am a stepmom and my dsd has always chosen to call me mom. Before dh and I were married, she would always ask and ask and ask (she was 6 when I first met her). I told her maybe she should try another name like Mimi or Candy or really anything else. She tried half-heartedly, but nothing ever "fit" for her.
When we got married, however, after we all had said our vows and were hugging, she looked up at me with tears streaming down her cheeks and she said, "Can I call you mom now??" And I told her yes.
For my situation, I decided to "allow" her to call me mom for the following reasons:
1) At her bio-mom's house, she was allowed to call her mom's partner "moms"
2) My dh and I knew we wanted to have several more children and we did not want her to feel left out or different in any way to "our" kids
3) It's truly the name she chose for me. It did not change the way she saw her bio-mom at all. I am just "mom" at our house.
Anyway, we always try to respect not only the parents in the situation, but the needs and wishes of the child. Had her mom not allowed the name "moms" we may have not allowed, "mom" for me - but since they were ok with it there, it didn't seem fair to dsd for her not to be able to call me mom at our house when that's what she clearly wanted. It's been 4 years now and she knows that she can change what she wants to call me anytime. But for now, she is choosing mom.