The mechanical LRD in the Tahoe's is a tried and true bulletproof one. I've had 3 with extensive offroad use in two tahoes and a silverado. The 21's is strong. But as mentioned, if you're looking for ground clearance, Z71 or not, the Tahoe is not your vehicle. While the air suspension can give you 2 inches in approach, it won't do anything with clearance of the suspension parts.
I drove a tahoe z71 and premiere, the z71 and the premiere having mag ride only, and the z71 having both mag and air. I bought the premiere because the ride was definitely better than the z71
You're absolutely correct, it's a crappy situation. Diminished value will depend on the insurance company's decision. Sadly, yes, it will show on the Carfax.