I can help with the first question. The simplest way to put your characters on the floor, not under or over, is to use "drop to the floor," Control-D shortcut.  This can get a little trick if they are wearing certain kinds of shoes (such as high heels) that may put a part of the shoe below the floor. You will need to zoom the camera in close to the foot and adjust the character's Body Y dial to make sure they are positioned where you want them. Do a quick test render if necessary. It can also help to turn on "Ground Shadows" in the Display menu.

Second question is MUCH more complex, and hopefully a true expert will chime in here.

 

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