One way to solve the scenario problem in a very compact way is to move away from "traditional" fantasy (which needs to be explained, and defined, and might not be familiar to unPlayers) and instead embrace some kind of setting that can be instantly understood and accepted by pretty much anyone. This includes "structural setting"... as in... there is THIS SITUATION, what do you do about it? Go!
It's basically a different way to do the thing that Brand_R friend does when larping ;)
Nothing has been explained in the traditional sense, and players might be imagining wildly different things, but as play moves forward and details are added here and there, what's important will emerge and crystallise. What you need is an effective way to handle the game conversation. Here I prefer gmLess solutions because they are tighter, more reliable and more challenging to design, but a decent set of instructions for a gmLed approach would serve the same purpose. No need to chain the whole game experience around physical combat, which to me is possibly the most uninteresting element of any game :P
Most veteran players (and designers) seem to seriously underestimate the ability of people to imagine stuff, given the opportunity to do so. It might not go in the direction one expected, but it might also still work amazingly and be a lot of fun. There is quite a lot to say about game that don't expect players to follow a pre-determined imaginary. Leveraging this quality in people allows designers to spare a TON of text that would otherwise need to be devoted to lore, setting, scenario definition, etc.