• The first method I want to know is the fastest way to stop the boat under sail, for when I am still the skipper. My first guess is tacking into a heave-to.
I suppose the first part of this answer should point out that there is absolutely no way to stop a boat, aside from tying it to shore. Bare poles, heave-to, head into the irons, will all leave some movement. In fact, in each, you are arguably in less control of your boat than if you kept it moving in most circumstances. Therefore, your choice is probably going to differ, depending on why you want to stop. Do you need a break and have a lot of leeway, then heave-to. Is there something you're afraid you will contact, then you shouldn't stop, you should maneuver in another direction and stay in control of the ship. When you're the skipper, you never get the option to fold and throw your hands in the air.
• The second method I want to know is the easiest way to stop the boat under sail, for when I have a guest aboard and need to teach them in a single sentence what to do with the boat if I go overboard. Something simple and easy to remember even if you've never done it. I normally assume just letting go of all controls would be the way to go.
This the bad news. Seasoned sailors have a tough time stopping a boat under sail and returning for a MOB. There is no single sentence that is going to effectively instruct a guest to pull this off. You should practice it yourself, in the event they fall over. Nevertheless, the first move is to turn into the wind. A non-sailor probably won't really know which direction that is, as obvious as it may be to you. Drop the sails, turn on the motor, get back to the victim and get them aboard....... ain't going to happen. Of course, a sailor may use the quick stop or figure eight method of returning to the victim, but if you're alone, the odds are very high that you will lose sight of them and have trouble even re-locating the MOB.
You're much better off understanding you'll need to self rescue. Always wear a pfd that will keep your head above water, even if you were knocked unconscious while going overboard. Have a PLB and/or waterproof radio clipped to it, so you can directly call for help. The portable vhf radios that have integrated gps are terrific. Water temps are a critical issue, depending on where you are, you don't necessarily have time to wait for someone to figure it out. They never will anyway.