I might have been better if you had started a new thread to post these questions, more people would have seen it, but never mind, I'll do my best.

It would help if we knew which version of Poser you are using. Questions sometimes have diffrent answers for diffrent versions.

1). If the "floor" is the GROUND plain than you can't lower it, you are stuck with it at the hight it is at, but you can hide it. Select the GROUND from the Props menu, from the Object menu select Properties, un-check the Visible box.

If you need a replacement for the GROUND plain, load a square prop from the Props Primitaves pallet, turn it on its side by xRotating it 90° using its paramiter pallet dials, yTran it to the desired hight, then scale it to the desired size.

2). These are probably "Ground Shadows", they are an aid to posing, not real shadows. You can turn off Ground Shadows, from the Display menu un-check Ground Shadows.

To print your render, first save it to a file. From the File menu, Export > Image, Chose JPEG in "Save as type" box and "High Quality 90" as the compression. Now send the JPEG you just save to the printer as you would normall do to print a file. This may not be the best way, but it should get round your problem.

3). I don't know.

4). You can set the Background Color to any colour you like. From the Display menu select Background Color, you will be presented with a standard box where you can choose any colour you want. Alternativly you can load an image of a night sky (say a JEPG photo) as a background. From the File menu, Import > Background Picture. Idealy the picture should be the same dimentions as as you desire for the finished render.