I'm planning 3 - 12 hour days of driving on the I-5 later this month. I expect 2 to 3 supercharges each day (not necessarily to full range) and a 6-7 hour "rest" for the Model S at night. Should I have any concerns with the Model S battery or motor overheating if I drive nonstop and then charge ? Would a 15 -20 minute break make any sense to let things cool down half way between chargers ?
You should be fine - the car looks after itself just peachy. The Tesla team breaking the cross country electric speed record certainly didn't give their car any breaks!
All the break in the middle will accomplish is to cool down the battery, which would cut down on range since it needs to warm itself up again. And lower range means you'll have to charge the battery to higher capacity - which is worse. So all other things being equal, stopping in the middle is actually worse for the car than not stopping. I've done a much longer trip on the I5 without stopping except at SuperChargers - San Francisco to Seattle in 1 day (19 hours or so). The car behaves itself beautifully.