Eco earth holds far more water than cyprus so much so that when wet you can squeeze water out. I personally don't like using cyprus as it is not well managed and the future it will be endangered (currently it is threatened in many places) Eco earth (coco coir) is a by product and 100% renewable. It is a sound environmental option.

The terracotta can work just fine but if you switch to plastic it will also work well and doesn't leak water to over the UTH and is easier to clean.

I am not a fan of reducing the ventilation it is linked to health issues. that method is basically 'sick building syndrome' I would avoid reducing air flow in a tank that has not super great air flow to begin with. (think grocery store open top freezers if they vented well with clients moving past all the time it would thaw. They don't because cold air settles and there is very poor air flow) I live where summer RH and winter RH is drastically different, summer is in the 40% winter is in the 10% I never had a problem.

Humidifiers work well although the monumental amount of cleaning the ultrasonic ones need (daily) to remain at low bacteria levels it not worth it. They also tend to drop the ambient air temps drastically when the kick on (summer mine was on 100% of the time) I'd avoid them and only use the evaporation ones they however cannot be directed straight in as they run too hot.