1. What does it mean by 'should I stop while I am ahead'? Do you mean 'should I stop the unit though it goes on running'? If then, just set the thermostat to 70 and just forget it.
2. Generally, freon doesn't go bad. It is almost inert except for open flames.
3. You can generally top up freon. If the leakage is severe, remove it totally, braze, vacuumize and then charge fresh freon.
4. Yes.
5. Can't say. May be improper design also.
6. Can't comment. I am not from your geography.
7. 60 to 65 should be good.
8. If the large round tube is non metallic, it can be insulation or may be a conduit. I don't know what a schraeder valve is. May be isolation valves(?)
9. Coil cleaner can reach places where your hands can't go. Mostly for inside coils. Can clean as well as keeps a check on biogrowth.
Regards,