The manufacturer makes no mention of whether or not the legs have to be deployed to use the pit. I would highly recommend contacting BioLite to verify this use.
Three of the four sides are mesh.
There is no good way to quantify the value you are looking for. Whether it will coat the underside of the covering is dependent on a a lot of variables including height of the cover, material of the cover, where the pit is located in relation to the cover and a ton of others. I would recommend reaching out to BioLite if you would like more information on the smokeless feature of this FirePit. Here is a good instruction video showing a quick set up guide and some "Do's and Don'ts" of using this pit:
https://www.youtube.com/watch?v=K5OCli6meFc
I am not sure I understand your question. Please email us your question at techsales@opticsplanet.com and we will be happy to help clarify.
This unit still has open flames, so using it on a wood deck is not really recommended.