Couldn't find what you were looking for at Fry's? Yeah, that seems to keep happening to me a lot lately too. I just keep going back... (glutton for punishment, hey that fits the theme for this game!) Honestly you might find a better mic at Guitar Center on W. Anderson.
Actually the green/violet pixels are detected by a half-pixel shift. The best approach is to double the speed that you are sampling so that you can detect 560 monochrome pixels. If you can do that then you can reconstruct the colors quite easily with a YU'V conversion lookup and a 4-bit window.
Great job! The black lines on the left screen is actually the behavior of the machine, it is part of the behavior of the graphics hardware. Frogger does this too. IIRC it happens when they reset a particular counter to multiplex the sprites in order to display more than two sprites on the screen at a time. I saw a bunch of demoscene folks explain how to code for VCS and I'll just leave it at this: It is even more bizarre than you can imagine. :-)
You can use mockingboard in a //gs but you have to set the slot to "your card" -- also for anyone wondering the Mockingboard has two AY chips on it, which is the same chip used in sinclair spectrum and Atari ST (which respectively only had one each.)