Fair enough about Gary Ross, but most of Hollywood is politics and money. He is infamous for not directing sequels, and money (or the lack thereof) has played a large part of that in the past. Take of that what you will.
As for the mockingjay symbol, it did start out as a symbol of rebellion. Had it been introduced in the movie properly, this would have been more clear. First of all, the mockingjay is a creature that was not intended to exist at all. By releasing jabberjays into the wild, the Capitol inadvertently aided in the creation of a new species when they mated with mockingbirds. The mockingjays thrived against the Capitol’s wishes, a rebellion in and of itself.
Second, remember that the pin originally comes from Madge. She says the pin belonged to her late aunt, Maysilee Donner, who died in the second quarter quell. Madge wears the pin at the reaping in remembrance of her aunt- a small rebellion of her own. (Madge is much smarter and more important than much of the fandom gives her credit for). She practically forces the pin on Katniss, insists that she wear it into the area because Madge knows the connotations of the mockingjay. She knows it is a symbol of rebellion, and she knows that Katniss is the right person to become associated with such a symbol.
So it really annoys me that the mj pin is introduced like “Here Prim, take this random pin that I just found and you’ll have good luck!” “Just kidding! But I’ll wear it into the arena anyway!” It’s just another one of the themes that GR did not properly integrate into the series from the start that could have made the movies so much more meaningful.