HDT, maybe Katniss doesn't do a great deal in Mockingjay, but what is she supposed to do? She's too important as a person to send in and let her go Rambo on the opposition. Considering everything she does do in Mockingjay results in her being hurt in some way, that helps to reinforce the notion that she is not superhuman. She's not much use as "the Mockingjay" if she gets killed fighting Peacekeepers in district 8.
I agree K isn't as good after THG, but I think that's also partially an illusion. THG is 2/3 action and 1/3 build-up exposition. By contrast, Catching Fire is only 1/3 action, and Mockingjay is too all over the place to be broken down neatly. I certainly don't like the way the Quarter Quell went in CF, and I thought her attempts at being the people's voice and an actual soldier were significantly below what she had been built up to, but I think saying she sucked after THG is a little harsh.
Drak, only Gale was in favor of blocking all the passages. That was his instincts as a tactics-first person. Nobody supported him on that notion, and even Katniss who inwardly agreed countered him because it seemed too much like a mining accident. The leader there decided to call it in to Command to let them decide, and they chose to leave the tunnel unblo9cked. The only thing that this incident proved to me is that Gale is willing to do Whatever It Takes, while the rest of the rebellion still wants to preserve some level of humanity.
Gale had also given up on Katniss before that point. There's a scene before the party splits up where he says to Peeta something about "that's the only thing I had going for me." I don't remember what that was in reference to, but it was something that no longer applied. Plus, he expected her to shoot him when he was in the process of being captured, which she failed to do. So at that point, it's not surprising that it's Peeta she ends up with, but I still don't think he would have disappeared forever.
Or maybe he doesn't. That's part of the problem of the ending. We don't actually know what happens for the mostpart. We don't know if the Capitol Hunger Games actually take place. We don't know if the rebellion government under not-Coin leadership is oppressive and diabolical or not. We don't know what happens to the few remaining characters besides the D12 victors, and Gale in particular is cast aside with one throwaway line.
My friend who lent me the books figured out what it was I hated most about Mockingjay: not a single character (good, bad or gray) ends up in anything resembling the situation they were hoping for. Most of the characters we are meant to like or regard as decent are dead. The ones that aren't are irrevocably altered and broken -- their loved ones are gone, their motivations are shattered. The only thing that led Katniss to the first Hunger Games was taken away in a completely random and senseless act. Peetah's love for Katniss was obliterated by his reprogramming. Hell if I know what Haymitch ever wanted, but he never comes out of his drunken stupor again. K's mom runs away to a new life apart from the ones and the things she loved; so does Gale. Annie has lost Finnick, the one person that brought her back to reality a bit.
TBQH, I'm really not sure what Collins was trying to accomplish with the ending. It's just so horribly, horribly blundered that while it's obvious there's an underlying message that she wants us to despise something, the only thing I end up actually despising is the work itself. That's really rather a shame, because I did like the first 2.8 books.