I think 66 backs up for more reasons than that. Though it is one of the bigger reasons for the backup. However, taking the long way around does run the risk of any problems at the Woodrow Wilson Bridge...

Alternate routes... hmm, you could from Centreville run up 28 and pick up the toll road (267) and take it to 495. IIRC, you can get onto 495N (inner loop) about a mile farther north than the 66 interchange, which cuts off a chunk of the Tyson's traffic (though not all of it). You could get off at Oakton and take 123 all the way over to the beltway, but wow, that would be a nightmare in and of itself. ()

As for the HOV lanes, the only benefit to using them is to take the HOV-2 ramp off of 66 onto 495, which is on the left just past the two right-hand 495 exits. Since it's HOV-2, there's often less traffic than on the normal 495N ramp, which can get really backed up (affecting the outer loop ramp, too; I often head that way and watch as people wait until the last second to merge out of the exit only lane for the outer loop).