1: Yes, but if you have crappy signal from Comcast, the splitter may be enough to do your connection in. Fortunately, they're cheap. The cheaper, the noisier, generally
2: See 1. If you have good signal already, you'll be fine. If your signal is marginal, the splitter will make itself known. You'll get weak signal levels at the cmodem, and snow/artifacting on the cable tv.
3: No, Maybe, and No. The wireless portion of the router might pick up interference if it's near other electronics. Electronics are generally designed NOT to do this, but nothing is perfect.
4: Ask your local buddy/ tv installer... they'll generally recoil and tell you no don't do that, but if pressed they might divulge a product that is less crappy than the others. Google might also help.
(5) Q:Is Wireless N good for penetrating walls? A: Not as good as Wireless G. G is slower, but it penetrates better. If your N performance/signal levels are crappy, try getting a G receiver, assuming your router can transceive on G, and give that a shot. Sometimes you get better throughput with G, due to too much interference with N, even though N is faster. This might clear up or reduce your dead zone. Check router specs etc before purchasing anything.http://www.smallnetbuilder.com/