As to I-495, when they raised the original 55-mph speed limit to 65, VDOT said they had never considered posting 70 mph on there. Insofar as I can recall, they didn't give a reason; they simply said that was how they approached the issue. Maybe the fact that I-66 is generally a much straighter alignment has something to do with it? I don't know, I'm just speculating.
I agree with you about what the speed limit on the Beltway should be. I generally set my cruise control to 65 mph in the general-purpose lanes and 70 mph in the HO/T lanes, in both cases for the purpose of keeping my speed down. I don't pass too many people at those speeds, and that's fine with me as well.