sure you can

you should be be able to use anything on the D300/D300s as long as it is design for nikon F mount

Refer to page 58 of your manual, it will explain all.

It is not possible to do a selective colour in camera, you will need some software for this.

Usually sold as body only but some stores might sell a package that includes a lens. Ideal lens depends on what you are planning to photograph and how much you are willing to spend. You can also consider other lens companies such as Tokina or Sigma.

You can't delay the focus because to initiate the timer you must push the shutter release which in turn focuses the lens. Why not get a remote ?

Using faster shutter speed will make it worse. If anything, you will need to use slower shutter speed and a tripod, but that will cause blurring of motion. With auto ISO setting, the camera is shooting at 3200 ISO consistently because the camera is desperately trying to compensate for the lack of adequate lighting in the theater. You have two choices. Use an external flash (SB-600/700/800/900/910) if they allow it and get closer to your subject (assuming you have slower kit lens i.e. 18-105mm VR or 18-200mm VR) - closer you are, better your chances of capturing blur-free image for a given variable aperture zoom lens. OR Invest in faster Pro lens with constant f/2.8 or fast prime lenses (usually cheaper), i.e. 35mm f/1.8, 50mm f/1.8 of f/1.4 These lenses allow more light to hit the sensor. Shooting in the dark theater is a challenge, even armed with capable camera such as D300s. In order to get the results anything that resembles what you see in magazines, you really need faster lenses with high ISO settings if flash is not going to be used.

Try using a different card.

Nikon does not make one, they have an external mic but not for a lapel.

No! It has a buffer that temporarily stores photo while it is written to memory card.

D300s has movie mode, 2 card slots, info button, 7 fps versus 6 and a few other minor changes. Picture quality should be identical.