I have problem with YafaRay engine too:
when I start rendering, a lot of text is logged into the console and then nothing happens - nothing is rendered.
But sometimes it works without problems.
I think it gets stuck before it has log this
[16:33:14] INFO (+ 1s): Environment: View number = 0, view ...

(sorry for my english)

1) layers
- if you have more layers, then JTabbedPane with tabPlacement on top is not best solution (my opinion) - left or right will be better. Or, even better, special table (like list of furniture) - with column: name, elevation and visibility.

2) floating panels ...