Well, first if you want to keep the humidity high keep a dish of water over the heat unit, that will incease it, And reduce the amounts of vents that the humidity can escape. In the US there are some automated misting systems that come at a high price,
You could also put an air stone in the water dish with an air pump like the ones for aquariums. But if you have low ventilation that will increase the humidity to dangerous levels for your snake. My Friends keep jungle carpets and I don't think they need very high humidity, 60% should be fine. But dont quote me on that.
Either way, Post some pics of your cage, You might just have to much ventilation for it, all of your problems could be an easy fix.
Snakes? I just like to teraform!