Dr. Kev.
4.Do any airlines limit the number of flying hours for crew in regards to radiation?
Not that I am aware of. I am also not aware of any need to.
Not true we here in Asia have a monitoring programme and crews are kept off the polar routes if they get above 6 something or others.
In addition to what arcniz mentioned. The affects of cosmic or solar radiation on humans has never been studied. All of the "Limits" suggested for flight crew are based on other forms of radiation. So basically we are flying guinea pigs.
FG