idler rumble, heat, and record wear--a connection?

Replace your idler wheel. I wouldn't say the warmth will cause any problems. Is the ROK in a massive plinth or a box?

Massive ply 30 lb plinth. It has the General industries motor. I havent measured the temp, but I'd guess about 100 degrees F.

Is it an L34?

You might try a heavier oil and new rubber on the wheel because it is possible to get rid of the rumble. Check the idler wheel shaft, too.

I wouldn't worry so much about the heat, although there should be a way to devise a deflector, if it really bothers you.

At 33 rpm, assuming no significant slippage of the idler wheel against the platter, and assuming there is some sort of mat between the platter and the LP that ought not to be a very efficient conductor of heat, unless it's made of copper, I cannot imagine that it could be "normal" for the LP to heat up as much as 30 degrees F during one 20-minute play. Something is amiss. I am thinking about the bearing assembly in particular.