I agree with your anticipation that the transition will be complex, but then what in life isn't?
It's completely fair if you have a need to know more about your "metamour" and to tell your boyfriend you'd like to get to know her. Often, knowing the other person can do so much to relieve jealousy and insecurity.
Have you asked him why he's so reluctant to tell you about her? It's not like you're asking for every last detail, just that you need to have an idea of how their relationship is growing and what that will mean for your own relationship. That's perfectly reasonable.
Some polyfolk refuse to do poly when they aren't allowed to meet the other people. I myself am that way. I don't like secrets and I like to know why people are trying to hide things from me. For example, are you sure that she knows he's in a committed relationship?
Agree completely: Out of sight is not out of mind. If anything, out of sight gives a person all the more reason to worry and be anxious. There's nothing as scary as the unknown, and reality is never as bad as the horrors my imagination can come up with!
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker