The two of you already had your time/chance. It takes a while for feelings to cool off.
She is engaged and lives with her partner. What you may want to do is be a gentleman and let her live her life. She may be confused about her feelings for you. Something had taken place in the past that led to the divorce (whether it was you who had the infidelity or her) Why would you want to get into the same possible situation again?
It is all right for you and her to keep in touch and remain friends (you have had a history together and a family) What may be unhealthy is to try and rekindle something back. It is possible for things to appear more appealing now that you're separated (but think back about the time when you were a couple. When there is minimum friction in any relationship things appear wonderful) As time goes by, those dynamics change and people fall back into a pattern they're most comfortable with.
She may be asking 1/ out of curiosity, 2/ trying to stir up jealousy because she's living w/ someone and if you were to say "No you were not happy" she could use it against you, et. It is best to leave it. If she is not happy 100% (and no one ever is always at 100%) you do not want to undertake the chance of trying to make it better for her. What if you fail? She can not expect you to fix her own life. She has to decide what she wants, be clear about it and not try to give you mixed messages. You were burned once already.