While those are good emergency alternatives, i would suggest once you can to have a mechanic shop replace the bleed screw as this is important to ensure you have all air out of the caliper and it is located in the position it is for a good reason. The above 'fix' may be good for emergencies, yet i feel it is best to use the proper location when you are able.
Bleeding from those points will remove air from the brake lines, but not behind the pistons.
Air in the system usually gives this symptom: When you press the brake there is a delay - because of the delay you tend to press the pedal further down so that when the air is finally compressed and the pistons start to move, the brakes will apply much more strongly than you intended.
Often this effect gets progressively worse but does it slowly. Driving the car regularly, the driver will learn to compensate for it, and it may not be noticed until somebody else drives the car.
You say that you have a poor pedal - but the pedal is just a valve with no real 'feel' or anything that can change as such.
Whether it gets air out of the pistons really depends on which alternate you use. For instance, if you use the alternate bleed where the line goes in to the piston (right hand picture, lowest alternate point, just below the legitimate bleed screw) it will get air out of the piston (or the vast majority of it, anyway).
In any case, I agree that it is always best to get your bleed screws replaced and in working order. In fact, I need to do that for exactly the bleed screw mentioned above on my left rear caliper. Some idiot at some point in the past put a bolt, sans head, in where that bleed screw should be. It's not a broken bleed screw since there's no central hole in the thing like a bleed screw has. On top of it all, some idiot (probably the same one) managed to snap off the head on the fitting that goes in that same spot that could be used as an alternate bleed point. Very frustrating!!