- when would you do this, and why?
The only time I would actually
plan to worker steal is when playing on Marathon (massively effective) or a cage match situation (alone with single AI you intend to choke, 18 civs on a Duel map, etc) which favors foreknowledge of the map set up, at least the script and size. In more normal or random play, worker stealing is mostly opportunistic with chief factor being proximity and even then can be weighed against just an early rush designed to take settled cities instead.
Stealing workers is so stupidly good on Marathon I would probably always seek to do it on Immortal, if at least you don't suspect you're on like an Archpelago map or something with no victim. Each worker stolen saves you up to 30 turns of city idle, speeds your improvements to not-agonizing levels, and can really, really cripple AIs as they don't tend to use their workers effectively in the first place, which is exacerbated on Marathon.
If you are alone(ish) with an AI who isn't going to be useful for much, choking them can work as a method to get something out of them worthwhile by denying them land for yourself to take, stealing workers by killing settler parties, etc. Stealing workers/settlers regularly and getting peace between really ruins an AI as they slavishy replace each growth unit and hemorrhage combat units in the process. Personally I like to choke a lot playing as Pacal, Sitting Bull, or Charlie as they are quite suited to it with Holkan/Dogs/fast PRO archers. It's also good for Charlie because you typically need to do some teching anyway to give a worker something to do, which you can do while building a warrior (for initial steal) and then archer.
In 18civ Duel maps stealing workers basically gives you the edge to roll a bordering AI, and start the insurmountable snowball. Especially if playing a civ with a resourceless early unit. Whoa mama.
- how do you get out and make peace?
Raise relative power or get war success by:
-killing a unit (scouts are popular, can also defend against AI units successfully)
-increasing population (settle or grow a city)
-build a barracks
-build war units (not very effective in personal experience, as the AI will too)
-get lucky and have the AI lose units to barb combat
It depends on the AI, lower peaceweight leaders can be very vindictive and hold grudges. More peaceful leaders tend to tap out easier. Not sure what exactly it's tied to (unit courage?).
would you still open with a worker (so that you have 2) or in any circumstances would you open with something else (and rely on the stolen worker)?
Outside of the scenarios I listed above I probably would still plan to do a normal open, especially if on a civ with a Hunting start. If you start with a Warrior you always just poke around and see if you can get away with it. But relying on a stolen worker is quite risky especially if you have a good plan for your start.