I think joining up with her is a important part of training her. I was waiting to get her roundpenning well before I put her saddle on but she is fine with the bridle and saddle blanket. I have put weight on her and done groundwork on her such as moving her front and hind end around, sidepassing, backing up, and foot work.
She is easy to get moving, but she gets tired fast and stops until I can get her going again. She will slow if I stop but won't come to me. I turn my back but she just looks away. I tried to answer your questions but I may have left some out.
I agree that join up, or whatever you want to call round pen work, is important, though certainly not essential. It's possible that this mare is of a rather sensitive and what I might call shy or uncertain nature. When you have her moving, she is fleeing you and might not be moving with a connected response to the level of pressure you are putting on, but rahter just running away, like I said, "fleeing".
Ok, some people just want the horse to flee them, round and around, until the horse is tired enough that it kind of "begs" to be allowed to stop. Then it's supposed to turn toward you and come to you. I think that's how it goes, no?
But what you might be looking for if you were working with me in the round pen, (In my limited knowledge) would be for the horse to be looking at you for direcction, and that would mean not just wanting , beggin you to stop driving it and allow it to stop, but for it to be looking to you for direction on HOW fast to go, and when to slow down, and turn around , etc.
You can work on those things without ever having the horse beg to come in and "join up" wiht you. YOU ask her to move forward at a walk . If she runs off, then stop and turn her, and try again. Until she walks. This mean that YOUR body language has to be saying to her "walk", and not "run away from me". It's hard to do this without someone showing you the way. It's very hard to learn from books.
Anyway, you work on getting her to go slow/fast/change directions, and when you stop moving her, she should at least stop running around, as you say this is what she already does (good). If you would like to draw her in to you, you may need to do more than just stand there, beuase if she has a sensitive and timid nature, she may not be confident enough to come in without more "invitation" from you. Such as, you back away from her, or move away and toward her hind end a bit. She'll follow you with her eyes, and where her "mind" goes, her feet will follow. If you stop, you back up, you move around to her hind, you turn you body somewhat sideways, (all ways to invite her) and she won't come, then ask her to move on and try again.
It's all about getting her to think ON you . If she is looking at you , thinking about you , maybe, but not moving her feet, then help her to move her feet by other means of "drawing " her, and if that won't move her feet, then drive her forward a bit, and try again with the stop and draw. And if she starts to step toward you, then you kind of "peel off" a little and she will likely step in and follow near your shoulder. THAT is enough of "join up" and time to go on to other things.
I wish I could help you but I wish you the very best in your efforts and hope my explanation can make some sense. It's hard to write , much easier to show.