Yes, draining the radiator will drain the coolant out of the block (most of it, anyway). It's a good idea to drain and refill your coolant before you restart the motor, because there are probably bits of gasket material in there from when you scraped the gasket surface clean. A good flushing is recommended.

It's also a good idea to change your oil and filter before restarting, because some of the coolant that was in your cylinders may have drained down into the crankcase.

About removing the sensor: I use Liquid Wrench and WD-40. Doesn't work every time, but usually does. Maybe someone here can advise as to whether applying some heat to the area where the threads are would ruin the sensor?