You're right, it does appear you have little to lose in going to lunch. And maybe much to gain. She did say yes.
XW is entitled to her thoughts, opinions, beliefs, etc, it is difficult for me and for many of us I expect, to understand.
Should something ever redeveloped, who says I'm not open, I'm not sure I could open my heart again wondering if it would get kicked out my chest again. I do love her, I always will. She does not believe this.
But life must go on. Jaded, not so much as disappointed, more at myself.