1. Knowing all this, how could I have addressed this? I truly do respect her need not to feel monitored. At the same time, I felt deeply disrespected and insecure.
I understand your discomfort... .it's not cool to see your partner flirting with other people.
However, I wouldn't worry excessively if she is not hiding it from you. The more you react negatively, the more she will do it. IF the flirting is something you cannot tolerate, then it needs to be addressed.
Some people are flirtatious, they enjoy the attention, the excitement. Others might feel the need to fill a very uncomfortable void. Or it might have to do with their insecurities and fear of being alone (they string people along in case their present r/s fails). Others might enjoy having open relationships. Or it could be a way of gaining power, creating jealousy.
Do you see your partner fitting into any of this?
Is this something you can put up with?
In the meantime the best advice I can offer is to work on yourself (insecurities), become comfortable with who you are and bring the best of you into the r/s.
2. If we resume communication in the future, how can I address this? It seems to me that bringing up old conversations will not be helpful. I would need to state it as "this is my boundary now, for any person I choose to be with."
If she doesn't flirt, I wouldn't bring it up.
But if she flirts inappropriately and you feel disrespected but you don't bring it up, then you'll be acting from a place of fear (fear of creating conflict, fear that she'll leave, etc). Any decision you make from a place of fear won't bring good results in the long run, plus, you'll be reinforcing her behavior.
Also, notice how she gained power over you. She made it clear that she will do whatever she wants (no negotiation) and when things didn't go her way she ended the relationship (unilaterally). Now you are left questioning yourself if you can bring up a conversation and you fear that she will get mad, that she will leave.
My advice is that you stand up for yourself. I can't guarantee that things will work out with your gf but I can guarantee that you will feel better about yourself if you defend your beliefs and values.
You can choose the appropriate moment and state things calmly, lovingly but assertively.
Boundaries, sure. How would you state this boundary?
Recommended reading: "Getting Our Values and Boundaries in Order"
https://bpdfamily.com/content/values-and-boundaries