Here's a post I wrote to you yesterday, and THOUGHT I'd sent before leaving the house, but clearly didn't.
I'm sending it now, just in case it gives you any other thoughts. It sounds like you did a lot of what I suggested, even though I never sent it. So your instincts were very good! Must be that "teacher brain"!
Horses can kill a dog with one kick, even if the horse just meant it as a warning kick. Horses are prey animals, and can definitely feel defensive about a dog running right up to them and they have no clear understanding of size, so a little dog can be as scary for them as a big one, particularly if they are not used to dogs. Then they turn their rear toward the possible threat. Dogs don't understand that this is the "danger end" of the horse, and particularly those with herding tendencies, and that includes havanese, will often run in to take a nip at the horse's heels. That's when they get nailed. I used to work at an Arab breeding farm, where the horses were not "cold blooded", like Icelandics, and neither were they dead-broke school/trail horses. Some of them were ridden regularly, but they were highly trained dressage and event horses, NOT used to dogs under foot. Or... they were brood mares who rarely got ridden, were tame enough to handle, but had even less experience with dogs. We had a Border Collie who lived on the farm, and twice she got nailed while trying to "herd" horses that we were leading to the breeding shed. The first time she got a broken leg. The second time she got a broken skull. If that kick had been at a SLIGHTLY different angle, it would have killed her. And she was a 40 lb dog, not an 8 lb dog. We also had one horse who would purposely pounce on and kill cats that went through his paddock, and I suspect he would have handled a small dog the same way. (although that behavior was EXTREMELY unusual... I've never seen it again, in over 40 years around horses)
Maistjarna is right, there is no way for us to predict what Shama might do. Unless you are ABSOLUTELY SURE that she will be under perfect voice control, I'd make sure that I arrived VERY early, with Shama and a long line. Spend as much time as she needs to get her used to working near the horses. (or within sight of them, anyway) Work closer to the horses than you think she'll be on course, just to make sure. Practice LOTS of recalls on the long line, with the yummiest possible treats.
ONLY if she is so good on the line that you feel sure she will come to you the moment you call, would I even consider taking her off leash. They certainly CAN learn this degree of control, and it's a really good thing to teach her. I would go, for sure, if only to work on this. If she doesn't seem to be able to pay attention to a level where she can safely be off leash, if it were me, I'd ask the trainer to help turn it into a lesson on THAT, so that you can enjoy lessons there in the future!
I went to a dog show last weekend that was held at a county fair grounds with a harness racing track. Kodi was not competing, but I had him with me. (I was watching some friends compete) I took the opportunity to put Kodi through his paces, including drop on recall and off leash heeling (well, he was dragging his leash in case of an emergency, but I wasn't holding it) just because it was very, very good distraction training. He was very good, but he's a LOT older and more experienced than Shama is.