Straight 50 probably would plug the leak.
No, it definitely would not.
You probably can’t get all the air out without using some specialized equipment, but you can get a lot of it out by finding the system’s high point and opening it a bit to bleed out the air. The high point may be a hose connection (radiator/heater/bypass/throttle body) or an actual bleed valve depending on the vehicle. When liquid comes out instead of air, close it up again.
I’ve had a lot of success with Bar’s Leaks over the years. It doesn’t always solve the problem, but more often that not it helps and it’s pretty inexpensive. I favor the old-style “pelletized” formula. While it may not be the only brand that works, it’s the only brand I trust to usually work and not plug up the radiator or heater core.