Not quite sure why you want to use the flying wing here. Or, at least, the types you've referred to so far. And I'm not sure it's right to talk about "the flying wing concept". There is a sort of flying wing that comes to mind, and that's the V-173 aka flying flapjack, prototype for a fighter. Although it's kind of overweight for your mission, it's one of the few aircraft which, according to reports, had the kind of takeoff and landing rolls you're talking about, and a top speed considerably greater than you require. However, the drawback is that you'd have to find a reliable way to drive those two props out at the edges of the aircraft. They're critical to this concept. Since you don't have to fly inverted, you may be able to get by with a better shape and less power. I'm thinking a long drive belt which drove both props might be good. You don't want one of the props to keep going if the other one isn't working.
I think you might find Zimmerman's patent for the V-173 interesting. Also, Zimmerman's two reports on "airfoils" (really wings) which can be found on ntrs.nasa.gov. (try a search on his name, and the dates which are in the 1930's). Lots of other neat stuff at ntrs.
A delta wing might be something to consider, particularly since there's more documentation on them than the other low aspect ratio types, and more than for other flying wings too.
A tricky issue with a low aspect ratio aircraft like this is that the cheapo design software won't handle it correctly. I'm not sure what the solution for that is, particularly if you can't build anything. (Like a scale flying model, perhaps.) But I think it's one way to approach your "mission".
I'm not sure being on the outside of an aircraft going 90 mph would be very much fun. And it will cost lots of performance.
If your requirement for a flying wing was relaxed, you might be able to do this with a cleaned up, somewhat overpowered, conventional ultralight design. A lot easier to design one of those.
Your mission spec makes me think of the Colditz glider! You don't happen to be stuck someplace, do you? ;-p
BTW, some other downloadable tools you might find useful would be XFLR, Xfoil, Profili and Xplane (Xplane and the more useful version of Profili are not free, the others are). But as I mentioned, they may not be very accurate for a low aspect ratio aircraft.