From what I understand the shifter is just a friction fit. You should consider an aftermarket shifter if you would like to go with a different knob.
Here is how to play a DVD while driving:
1. Insert DVD into DVD player.
2. Start the movie, do NOT leave the the disc at the DVD menu or you will not be able to start the movie after you release the parking brake.
3. With the movie playing, press the second from the bottom button on the right hand side of the DVD screen and the forward Seek Button. The screen should then flicker. If you get taken to a screen that says "cold reboot", press the same button combination AGAIN and it should go back to the DVD...
4. Release the parking brake, the DVD should continue playing.
You will not be able to control the DVD after you start it playing. If you try to, the screen will go to the same screen you get when you try to start a movie without performing the above steps.
As far as aftermarket parts goes, there is a pretty comprehensive list of vendors on the forum here:
Aside from the vendors there, I would also recommend Lund Cadillac in Phoenix. You might consider giving them a call to see what the limits for mods withing the warantee actually are. I know if you have their service department install your mods, they will continue to service the vehicle under the manufacturers warantee (although that obviously won't work for you considering you're in New Jersey). Their website is:
Last but not least, welcome to the forum!