The only way to guarantee a horse stays in a good home is to keep it in the good home you provide. Sorry, but that is just how it is. Otherwise you just have to trust the person you are selling to and hope for the best. People buy a horse, especially one they pay thousands of dollars for...they want to feel like it's their own. They want to buy the horse and ONLY the horse, not with a bunch of strings attached and they don't want to buy the former owner with the horse too. It's like marrying the man of your dreams and having your mother-in-law from hell move in with you.
Myself, I only sell to people I know personally. People that I ride with, people that I've known for years and people that I see socially on a regular basis. People that know the horse they are buying from me and people who's horses I know, I know the care they give and I know how they keep their horses. They may not have the exact same standards of care that I do but if their horses are healthy and happy I've still got peace of mind knowing the horse I sold really went to a good home. I don't have to resort to stalking to make sure the horse is cared for because I still it regularly, I get casual updates on horses I've sold over margaritas with friends, I get a phone call when there is a problem with the horse and I'm the first one they tell if they have to consider selling (I've bought a few back over the years too). They don't do this because of a contract, they keep me in the loop because I am friends with these people in and out of our respective equine worlds.
All these stipulations you plan on putting your contract are illegal. Once the horse changes ownership you have no say whatsoever in the horse's care and they can do as they please with it and if you actually tried to enforce such a contract it would get laughed out of civil court. Even buy-back clauses are a mere formality and there is no way to get the horse back from new owners if the people you sell to decide to sell to someone else. They have legal right to sell the horse to whomever they chose and your previous contract will not nullify the totally legal sales contract they enter into with someone else.
If you're this set on dictating this horse's care then selling isn't for you. Since you say there is no financial motivation to sell you may be better off doing a free lease so you retain control of major decisions concerning this horse.
I'll be honest, what you are describing is a buyer's nightmare. No one wants to deal with that seller who cannot, for emotional reasons, let go of the horse they are selling. You are the person most buyers avoid at all costs no matter how nice the horse is. Who wants to re-arrange their life for a day once a month, once every few months or whenever you want to pop in for a visit?
Here is how it feel from the other side of the fence, unfortunately I've got first-hand experience with that too. The family of the former owner of one of my horses stops in for visits with my horse on occasion. The former owner of the horse passed away (which is how I ended up with him). I've had the horse for about 6 years now. I get little or no notice when they come to visit which can be aggravating. Sometimes I'm unavailable and they show up anyway, I'll be at work and my barn owner has to entertain them. A few times I've been out on camping trips and had the horse with me and they didn't even get to see the horse. They've made sideways comments about his weight because he is thinner that when I took the horse, they think obese is healthy and I prefer to keep the horse at a healthy, trimmer weight. They do not like the fact I keep him barefoot, he was kept shod the entire time he had them and I've worked hard to get his feet toughened up so he can stay 100% riding sound with no shoes. They think he should be getting large amounts of sweet feed but I've found he does much better on a pelleted feed formulated for older horses (he's 25). They think he should be fed alfalfa but the grass hay I give him instead is more affordable and helps maintain the weight I like to keep him at. I like the people, they are a very nice family. We have differing opinions on what is the best way to care for this horse. Once every few months I find myself having to defend my decisions for this horse during spur-of-the-moment visits from this woman and her adult children, and running referee between them and my barn owner. They've never asked to take the horse back and I honestly don't know what would come of it if they did. I assume I would retain ownership of him since I've been taking care of him for the last 6 years and have a signed transfer of ownership but I've still got that hanging over my head...the what-ifs. It's a worry I don't really care for.