How risky is smoking one cigar (~30 minutes/cigar, non-inhaling) per week with regard to cancer?

Oral cancer risk. Even if you do not inhale you are exposing yourself to risk of oral cancer. No good can come from use of tobacco. Quit while you are ahead.
Difficult to say. It is unlikely to have a significant increase in your odds of getting cancer, but this risk has many components (past smoking, parents smoking, etc.).