1. No, they ride in the rain all the time.
2. It's not so much that the white shit (actually lime) is slippery, it's that it's unexpectedly slippery. Riders are generally quite good at going the right speed for the conditions. This just a new and unexpected condition they don't have experience with yet.
3. They did shorten one stage because of extreme heat, and there has been talk about rescheduling (both in terms of season, and moving the races into cooler parts fo the day), but the Tour is pretty tradition bound, so it'd take something quite exceptional to see that happen.