I don't think these "friends" of yours are friends at all, and I don't think they ever had your interests or your horse's best interests at heart at any point in this deal. :( I think them getting defensive about you calling out a vet to check her out is them being worried that some unsavory info will come to light and you will want to ask them for some money back. Perhaps they discovered at some point that she is actually worth more (based on bloodlines or performance history, perhaps) than what they sold her for and they are trying every trick in the book to get you to sell her back so they can resell her for an even greater profit? Who knows. Everything about what your "friends" are saying and doing is extremely fishy and I would stay far, far away from them.
At this point I would make sure you have all legal paperwork in hand that the horse is legally yours now and not theirs. Based on the vet's exam it's really up to you on what you decide to do at that point and if the horse is doing well for you and your family then I think you should keep her and definitely dump your friends. I think if she were to wind up back with them her fate would be sealed. :( I think with you she has a much better chance at a happier, healthier life, regardless of her worth or how much you paid for her. It may wind up being a bit of a loss on your end money-wise but again if she is working out well for you and you can still get many good riding years and companion years out of her then she may be worth it.