One step forward and two steps backwards ;-)
I think most people have experienced a trainer that either did not know what they were doing, or was so far up themselves, that they did not care about their clients.
It is a shame and it is disappointing... but it is also just a sign to move on.
People are in our lives for a reason, a season or a life time. When their job is done they move on from your life.
Your able to work with him at home alone, you have learned many new skills over the past year. I would continue to do what you are now. Have a lesson on Maggie (or who ever) and then go home and practice the lesson on Tango.
Pick your instructors brains as much as you can and discuss options to work on with Tango. If you go home and try X and it does not work, next time at work say to her... well I did X and then Y, S and W happened???
Where did I go wrong and how would YOU fix that........... OK...... Now how should "I" fix that... LOL
It is always nice to hear what the professional thinks and then adapt things to your level and Tangos ability.
If you take a step back and look at your training notes, you will see he was going really well (reaching from behind and working kindly for a month). Then she began tying down his head and it "worked" for a while, but has now created other problems.
It is an unfortunate fact that each time we prop something up, that it creates another possible flaw.
I remember clearly several years ago that a top qualified instructor (teaching other instructors) and trainer told me to use a 3 ring elevator on Zig (from an eggbutt snaffle). I told her I did not think it was a good idea and she told me "SHE" was the professional.
He hated it, then subsequently worked hollow and short. My hands are fine, my horse just hated that bit. I used it in a lesson with her and the minute I got home I took it off the bridle. Next lesson I returned her bit and told her he did not need it, he needed more leg!! So my short legs had to get more oomph... ;-)
Check your notes and see what you were doing with him when he was going well. Chances are it was simple and basic stuff, which is usually what works!
Don't over think each session, keep it simple and put lots of positive energy into your work with him. He needs a leader with a strong mind and you need to be consistent with him.
The bonus you have is, that you know where your going wrong. Work backwards, think of the end result you would like and then work out what steps are needed to achieve it.
To answer your question - I would leave it alone. No point in burning bridges. Don't chase her and keep trying to make contact. Currently she is telling you (by ignoring) that she is either, not interested or simply does not have time to be out regularly.
The reason why I say not to burn bridges is because you never know when you might need help in an emergency. Unless the person is really detrimental, simply leave her as a mentor that to associate with occasionally.
Good luck ;-)