Does frequent air travel and exposure to cosmic rays make you more likely to develop cancer?

No. Theoretically the atmosphere blocks cosmic rays and your exposure is increased in planes. But remember that pilots and staff are up there constantly each day and no passenger can approach their exposure with frequent travel. Pilots are not monitored because this level is still very low and their lifespan has not decreased. Otherwise who would ever want to be a pilot or stewardess.
No evidence. To date no evidence and osha has not restrictions on air crew who would have the most exposure.
No. Well studied, especially those in the industry (pilots, flight attendants). Same issue applies to living in denver. It's just not sufficient or sufficently penetrating to cause an increase in malignancies.
It is true that. people who live at higher altitudes have a higher risk of skin cancer. As far as air travel, there is a minimal increased theoretical risk due to marginally more exposure. As a practical matter, however, it is not significant.