Hey, Youko here. I'm one of the primary testers for Luna and the one who made one of the UIs featured on the page (the gold and black one).
Basically to answer your question, the reason it fills back up has to do with you setting a colour for the gauge. Basically it refills because it gets confused about the conflicting settings (and probably the "blind: true"???). Simply getting rid of all the crossed out lines should resolve the issue.
As for your second request, the cleanest way to accomplish that is by basically making a second "gauge back" image and layering it behind the gauge so once the gauge decreases, it will show through. Simply add another image component with your source file for the back with more or less the same rotation and position settings, and add it above your ActorHpGauge component on the lunatics item list.