So sorry to read this. Tough sometimes, to figure out what is best for everyone.
You are a kind person, and they I think are taking advantage.
Wonderful that Katie is getting good riding, but is there a cost savings to you for all this use of your horse??
Sometimes it helps me to mentally review my overall experience with a horse to help me focus on a plan.
So to review, you have a wonderful horse. YAY! Remember the excitement buying Katie brought.
Then the low of trying to find the best place for both of you, and moving a few times.
So you have a place now, with good and bad points. It's really hard for me to understand the boarding situation over on your side of the pond, because it is so different from here.
Maybe the owners and staff don't realize how important it is for you to ride your own horse.
What would happen, if you spoke to the bO and said you are thinking of selling Katie, because basically you are paying lots of money to keep her, for others to ride, and you don't get to ride.
I am NOT suggesting you really sell her btw, just something to push the people at the stable to help you find a path through this. I would bet they do not want to loose Katie, she sounds as if she is the "steady Eddie" for the students to go out with.
As for your family situation, I can only pray that they find peace and do not suffer.