It seems like you really want to see your marriage T solo for these issues but the only real thing stopping you is that you feel that you would hurt your old T's feelings if she saw you coming out of your marriage T's office.

You do not need to see a T out of obligation. You do not need to communicate anything to your old T out of obligation. I am sure that your old T would not be miffed in the slightest and in fact would see it as your private choice if you wanted to see your marriage T solo. It isn't about her failing, it's about you feeling that someone else is in a better position to help you now. If she happened to see you coming out of your marriage T's office, she might be a little surprised, but I doubt there would be any weirdness. If you wanted to preclude feeling that there might be, you could ask your marriage T to mention to her that you are doing this, and/or talk to marriage T about how you feel-- I am sure she would reassure you that your old T is a professional and wants the best for you.

Besides the advantages you've already mentioned, I could see that marriage T may be able to help you more efficiently with these solo issues because of her ability to "see" the dynamics and what troubles you in your relationship. She may be able to "connect the dots" in ways that someone who hasn't seen you with H could.

I do get the "I thought I was done dealing with my past." For me, it was almost 15 years later that my past decided to make its reappearance in my life. Obviously, you're a much quicker study than me

Anne