Sorry for the super slow response. I have been traveling without proper power and I don't log into this site very often. I'm sure you have already picked another shot but if you want to use something of mine in the future go for it! You have my permission.
Yeah whatever your comfortable with. Bums do it everyday, sometimes with nothing. I will be honest though, when in a big city the homeless are sometimes grouped up and territorial. Not saying don't do it just be smart about where you sleep. I recommend picking something that is not easy to climb up or get into. The roof in this picture was a huge pain to get to, there were squatters on the property but no evidence of them ever making it past the ground level. All of the standard staircases were removed so getting up required some creative climbing.
Its blasted rock. They used explosives to create this tunnel. I think it has do with what type of earth they are tunneling through. Since this was solid rock I guess they didn't need to shore the sides or ceiling. Normally when its clay, dirt, sand, or something other then solid rock they use cement to shore up the tunnel and it would be dug instead of blasted.