Highly qualified engineers collaborated on this with thousands of hours applied to reason and come up with a plausible solution. However, I'm sure they didn't think of everything. Who knows, maybe some layman ideas could provide a novel solution that they couldn't see, being too close to the problem?
The airbag approach won't work because the payload is too heavy. Due to the thin atmosphere, the parachutes will slow the payload down to 200 mph but no slower. Thus, retro rockets are necessary. Those have to be on the reentry capsule (lander). The dust problem was probably too pervasive to consider a "dust cap". Once you've got such a thing in place, it needs to be ejected. Maybe they could have invested more money on special servo motor controlled doors for each entry point that might be affected by dust? But that would add to weight and yet another slew of devices that could go wrong.
The tether is kind of a nice idea, in that there is an inherent buffer between the landing craft and the rover. You don't have to worry about internal cushioning of the rover against the inside of the crane, to guard against the inertial shock.
However, if dust is such a problem, why not have the lander touch down with the rover attached underneath? The lander would act as a canopy to keep dust from settling down on the top. But... the dust problem may be lateral as well. The only thing that bothers me is that Mars DOES have an atmosphere, albeit a thin one, and there is wind. Wouldn't a wind storm present the same dust problem?