The classic method for avioding damage to a wing during a bad landing is to attach the wing using rubber bands so the bands secure the wing to the fuselage. The forward bands go onto dowel stubs sticking out of the fuselage that are pointed forward. This allows the bands to hold the wing on in normal flight but duing an impact the wing slides forward and pushes the bands off the forward pointed dowels. The bands spring loose and the wing continues and impacts the ground in a more kindly way on it's own. You'll seen this style of wing hold down a lot on cabin style models. The rear anchor dowels are merely a sideways mounted dowel that passes through the fuselage and holds the rubber bands on.

You'll have to define "rough terrain". Keep in mind that a regular grass field to a model is like a full size light plane landing in a field of wheat or waist high grass where there's lots of big mole hills and roughness. Operating out of an area with rocks, very uneven dirtand other hazards would not be a realistic scenario at all. It would be like a real plane being asked to operate from a boulder strewn field that had just been bombed. You can compensate to some extent with oversize wheels but in the end the field needs to at least be a realistic attempt at a grass playing field or the model will just flip over.

Padding an egg to avoid damage is as simple as molding or forming a block of foam around the egg so any pressure is distributed over the whole end of the shell facing the impact. With such a molded bed and a hard box shell around the inner bedding you can drop the box from the top of a house and the egg will still be intact. Mother Nature did a really good job on the basic design. You just need to ensure that the forces reaching it are spead out by using a firm material for the bedding.

As for the plane you don't design for high lift by using some sneaky feature. There's really only two factors involved. Keep the model light and make the wing big enough to lower the wing loading so it can fly slower. You also want to use an airfoil with lots of camber, up to a point, so the stall speed is lower. But the two key elements are the low weight and big wing area. If you don't have those the higher camber airfoil won't be enough to save the day. And don't worry about crash damage. A light but intelligeintly designed model will be tough and resiliant because it carries less kinetic force to do harm to it's own structure. Light but well designed models will bounce instead of busting. But you need to know where to build in the toughness and where to keep it light. The wing spar and the nose of the model are two areas where you can justify using a bit more weight for tougher materials. If you can keep those two parts strong then the rest can be quite light.</p>