About 150 deg.
Two separate systems. The heat is a closed system and isn't tied to the house water.
Typically a baseboard heating system. Consisting of a boiler, pump, piping, and a heat exchanger like aluminum fins.
If it is a hot water system and not steam, once it is properly filled & bled of air no more water should be needed unless there is a leak. Regardless of whether it is steam or hot water it is a Boiler not a furnace.
A baseboard heater runs through a process of induction and release, in a manner of speaking. That is, the burner will heat the water; the water will pump through the system; and once all is up to temperature, the burner will shut down. However, the metal of the pipes and the water itself will not cool right away; it will remain hot for a time and cool slowly. The closer the baseboard is to the furnace itself, the longer it will remain hot.
120-160 Degrees Fahrenheit
No, PVC is not rated for 180 degrees which is the average operating temperature for baseboard heat
Yes, 1/2 inch supply to fill boiler will work fine. Piping going to baseboard heaters needs to be sized properly. 1/2 inch piping to baseboard heaters not recommended. Most hot water baseboard heaters piped with 3/4 pipe.
Yes. You might want to check to see what recommended temperature is for water entering the baseboard units and insure the water heater will meet that. Something else to consider is that there will need to be a pump in the water circuit to circulate the water. A tank type water heater would also work to store hot water until needed by the baseboard units in an instance where heat might be generated at a time not needed by the baseboard units.
Your home heating system may use a furnace, boiler, electric resistance, hot water baseboard, radiant heat, geothermal or other system. Look through the phone book for a professional right away!
Your breakers
Answer No, if the current to the baseboard heaters is off they can't run or heat.
Baseboard heaters depends on convection currents in air to move heat around a room.