1) You don't give any figures for how common it is for doctors to get invited to a conference that is an excuse for a vacation. And I suspect you don't have any.

2) Even if it's true, there is a limit to how you much you can influence a doctor with such things. You might get a doctor to adopt some new drug which isn't as good as it should be, but you can't, for instance, get the doctor to oppose vaccination or convince him that saturated fats are healthy. It would be impossible to get doctors to oppose chiropractic this way unless there was already obvious reason to believe it's pseudoscience.

Cigarette companies had plenty of money and tried to buy as many experts as they could. The best they could do was get an occasional scientist or doctor on their side; they came nowhere near convincing the whole medical profession that cigarette smoking is safe.