All good questions!
To your first item, the number of doctors with a direct financial incentive to push drugs is relatively small (and, at any rate, troubling), but the reach of drug companies is much larger than their direct payouts. I am afraid that the biggest fear has to lie with the doctors themselves. Survey after survey shows doctors saying that advertising doesn't influence them, but the drug companies don't spend all that money just to amuse themselves.
I don't think "ask your doctor" ads are an attempt to undermine confidence in professional opinion so much as an effort to persuade patients to bring specific issues up with their doctors. Patients are often unwilling to ask questions about nonspecific or seemingly random issues. I'm not defending "ask your doctor" ads in general, but I think awareness campaigns can be very reasonable; the "ask your doctor" ad campaigns are aiming for the awareness dollar (in sometimes cynical ways).