[300SLR]

A 1mm radius on the outside of a tyre doesn't sound like a problem, could be an advantage. Some racer put that sort of radius on deliberately, it helps prevent the rear end hopping about in corners which can happen when the tyres have a square shoulder.

Wearing away the diameter of the tyres is the problem, how quick this happens and how little ground clearance you can run down to depends on a number of factors. Check out the ground clearance under the rear end and see how quick its wearing in YOUR conditions.

 

[300SLR]

If you have separate 50 lap races, then its probably a good idea to clean tyres between races. However, in non stop longer races, its questionable how often its worth stopping to clean tyres.
Stopping to clean tyres takes time out of your race, it's only worth doing if it makes you quicker for long enough to make up for the lost pit time.
That all depends on conditions so there's no universal answer.
In a lot of conditions 50 laps is way too often to stop, but as I say that's not necessarily true in all conditions.