This is how all that reads to me:
I am willing to meet because
- She and him have agreements between them to meet, which I think is fair. After all, he is *her* significant other first and foremost.
- She wants to meet me to put name to new metamour face. See about building trust as a meta and maybe alleviate her worry / jealous of the unknown?
- I am interested in meeting her; she has a doctorate in the field I'm going into.
I am not willing to meet because
- I am several years younger than both of them, and I feel that effects the power dynamics of the situation.
- I am also afraid that she will really hate me after she sizes me up, if she doesn't hate me already.
My question to you is... Which "inner voice telephone call" are you willing to answer at this time?
- The one speaking to willingness to date this man, allow him space to meet his agreements with his other partner, build some basic trust/manners with this woman who is now your metamour?
- The one speaking to ageism/lack of confidence and fear/worry?
Which of the two helps feed your "I can handle things in my life" bucket?
You are the captain of your own ship -- you get to take/make the call based on where your willingness happens to be at this time. Neither is "right" or "wrong."
Just... "at this time where is my willing at?"
If it were me? If I wanted to continue building relationship with this man? I'd go meet the new meta. It's not a big hardship thing to ask. Just a "meet & greet?" I do not find that unreasonable. Nobody is asking me to be her best friend or anything.
A reasonable kindness for to my Sweetie's Other Sweetie? It is also a kindness done to my Sweetie. Is this Sweetie worth the trouble or not? Only you can answer that.