Dan, Brazillians usually shed in one piece. If your Brazillian did not, then it usually is because the humidity is too low. The whistling usually also has to do with low humidity. I generally only cool snakes that seem perfect. If I had one that was making noise, I probably would wait to cool until the snake seems perfect. The reason I would wait, is because I have had perfect snakes that developed slight RI problems during cooling. My advice would be to wait on cooling, and focus on raising your humidity. Besides, I know people that sucessfully breed brazillians by cooling any month from September to December. My point is that there is no rush. You can try in December if things seem right then. The humidity should be at least 80-100% during the day. For future reference I have also included the answers to your questions.
1. I am going to separate the snake, so should they have access to bathe during cooling?
Many people have success breeding Brazillians both with large water dishes, and small. I cut the humidity to around 55-60 percent during breeding, and so I remove the large bathing tub and replace it with a 5 inch water dish. Only during shedding do I give the larger tub back.
2. should I continue misting during cooling?
Only if the humidity drops below 55%. With the lower humidity, you must watch out for RI, and if the snakes appear unhealthy then discontinue cooling and resume normal humitity and temps.
3. should the snakes have access to a "hotspot" during cooling?
I offer a hot spot during cooling that stays around 88-90. I only offer the hot spot for 4 hours daily.
Let me know if I can help in any other way. Howard.