Is it safe to replace a crushed car roof?

absolutely. the roof is just another panel. its an outer skin with reinforcements on the sides, front, and back. you can replace the roof and the damaged reinforcements (and any interior damaged items) and the car will be as good as new.

Its a tough one to answer. Basically the impact of the tree limb may have sent a shockwave through the car and damaged it in other areas. I think it depends on how bad it hit it. If it was a small branch and left a bad dent it would be OK, but if it was a large limb and the whole roof caved in then the energy of the impact may have travelled down the pillars and into the rest of the car.

I would get a mechanic to look over it if it is serious as it is possible that the impact may have weakened the rest of the frame.

Sure. It is definitely still safe. I would take it to a body shop and have them replace the roof. They will tell you if anything else needs to be done to maintain the safety value of the vehicle.

Yes it is very safe as long as it is done by a professional shop that will not cut corners plus remember that by repairing this it will bring down the value of the vehicle when it is sold, this must be disclosed......

If it's done correctly, then yes. After all, roofs are welded in place while on the assembly line.