How stable is the tunnel if not boring through bedrock? Frankly, I don’t know, but wouldn’t that be a consideration?
Won’t be long, they’ll be finding bodies down there also.
They have already thought all that out. If they tunnel underneath the English Channel, they can do this.
Concrete. Lots and lots of concrete.
Vegas already has a network of pedestrian tunnels that are great for avoiding the direct heat of the desert sun, and the vehicle traffic on the Strip, if you’re not interested in sightseeing. They also have a lot of small retail shops in the tunnels to accommodate the foot traffic.
Sightseeing in Vegas is better at night, when the Strip is lit up, and it’s (slightly) cooler.