Hi there, Derek. I can probably answer #2 and #4 for you.
At first glance, the black and red supreme avatars do appear equal to one another. Between the attack bonuses, ability scores, and saving throws, its hard to pin down a clear victor. When looking at the MM black and red dragons, you can see that the black is second powerful of the chromatics (of course, the red being the most powerful). What happens on the way to supreme avatar status, is that a dragon needs to hit the Colossal size mark. While the red does this 200 years after reaching great wyrm status, the black requres at least 600 years. This gives the black more bang for its buck since every 200 years means a stats increase across the board. Thus, by the time a black reaches eldest rank, its scores are on par with the red. I'd bet on the red in a fight--compare the pitiful d4 damage of the black's BW vs. the red's superior d10.
As for axing an automatic BW for half-dragons, well that's another story. When designing the half-dragon racial template, I wanted to break ranks with the MM version (indicating the WotC writeup was a quick standard one could use in a hurry or if a PC was magically shapechanged, but it was not a true
half-dragon). Maturing on their own, I thought half-dragon characters should have a choice as to what abilities they wanted to concentrate on as they grew, breath weapons included. That's why there is a BW feat (plus the ones a character can take after it), allowing you to take it at your leisure or not at all. It also gives half-dragons some unpredicatability.
Hope that helps. Joseph will have to answer your other questions. Thanks for your interest!