The doctor, when challenged to roleclaim, can not claim he is a doctor. It's unfortunate, but he has to lie at that point. Whether try to parry the challenge or give himself the false role of a townie, he has to lie. The doctor acts like scum in behavior, but not in motive. It's usually hard to differentiate the two, but I don't see a way out of that lie.

I only lie in a game if I'm scum. If I think someone is scum, I won't say I'm a tracker and tracked XYZ going somewhere and killing ABC. But hey, I might have used that idea, but it's been more than a year since I played with you. Dtewi is all growned up.

Well, let's look at it this way.

Doctor claims townie. Town turns to lynch a scum. Scum claims doctor!!!

what do you do now?

Marvel at the fact that the "doctor" doesn't die the next day. Then promptly lynch him. 

except by then the real doctor has been picked off because he probably didn't believe the scum doctor claim and said so. Also, we've had plenty of games that can easily expalin why the person you would think die doesn't. I'd only subscribe to that tactic if we knew we were playing a vanilla game with only 1 doctor and 1 cop.

This is all based on the assumption that a scum would role-claim doctor. In a vanilla game, they would never ever do that. In a power game, it becomes much more dicey, but I think a doctor for a scum is a good trade-off.

Of course. And I edited my reply above.

The doctor has to lie to save his skin so that's not really a good strategy, "Lynch all Liars."

No he does not, I'm for lynching anyone who asks "Are you a doctor?"

Or by the off-chance that the doctor is asked to role-claim?

Why on earth would the doctor be asked to roleclaim?