A built fortification retains a floor on the BOTTOM. The top's irrelevant for this outside of fliers and suicidal crossbow dwarves.
My understanding is that
all dwarves are suicidal in the current version, at least after they've had a drink. You want to prevent your civilians from deciding to join in the fun, presumably. (Or maybe you don't. But if you do, then you need to physically stop them from climbing over the fortifications and dropping down into the action. Ergo, the ceiling over the fortifications.)
All windows and fortifications are transparent. All walls and floors are opaque. The material is irrelevant. I believe that grates (both kinds) are transparent as well, but my experience with grates is pretty limited.
Dwarves will not sit in chairs unless they are performing a job that uses a chair (Eat, Update Records). At all other times, they are standing up, or lying down, or in a transitional state between those two (e.g. just got kicked and hasn't fallen all the way to the ground yet). Hell, for all I know, they may perform the chair-related jobs by standing on the chair. Of course, this doesn't stop you from installing a bunch of chairs in the meeting zone and pretending they're being sat in. If nothing else, the dwarves can admire the fine +chair+s as they mill around.