I hate to tell you, but unless you rig up some kind of automatic humidifying system you'll never be able to maintain proper humidity levels in an enclosure that big. Misting is not a long term solution. Works great for Green Tree Pythons, not so much for BPs. Every BP I've ever had shed in one piece more than 90% of the time. Any thing else is unusual and a sure sign your humidity is not right.

The reason your BP was climbing up and into the light was to get warm....another problem with the type of set up you have. In an enclosure that large maintaining proper heat gradients and warm/ cool hides can be problematic. Are you using any type of heat other than over head?

Finally, BPs are not terestrial snakes. Younger ones can be quite adept at climbing, but again, it's not a long term solution. Healthy BPs become quite heavy bodied as they grow into adulthood. In my experience, 3000 gram BPs don't climb very well at all. But then again, Mom Nature didn't design them to.