Replacing light fixture and main breaker?

The light switch will break the circuit so that there won't be voltage at the fixture when the switch is off. So, yes it is safe. However, it is possible that someone may accidentally turn the switch on while you are working or there may be other wires in that junction box that are not "dead". You don't need to turn off the main breaker to the house, but you should turn off the breaker that provides voltage to that light.

It would be much better to label your breakers at the box and then turn off the breaker for that circuit. I've been hit by voltage coming through a light circuit that I thought was de energized at the switch. Better safe than on your butt or worse.

not really, because you still have a hot wire in the j-box where the light fixture goes. If I were you, I would turn off the breaker.

yes just make sure its the wright breaker