Cancer is grounding until favorably resolved and after a waiting period. Polyps should not be grounding, but get them removed. Get some advice from an aviation medicine consultant. These guys are good, other folks can recommend other services:
http://www.aviationmedicine.com/
You don't need a medical to do sim.
Airlines typically employ qualified line pilots, medically disqualified line pilots, and non-line instructors on the sim.
The qualified line guys are very expensive, but they keep the training program aligned with operational reality.
The grounded line pilots may get line pay, or they may get instructor pay only.
The non-line instructors are hired off the street and usually get paid less than the line guys. They usually prefer guys with time in type (often retired) but I have seen random low-time guys doing it also.