Do you know that the separatist taking credit are both A: separatists and B: the ones who shot it down? For all we know they cljld be plants for disinformation. But even if they are the real deal, nnothing has linked them to being the ones who shot the missiles. For all we know, they could be cheer leading something someone else said who could have been a Ukrainian plant. Their bragging about it is not evidence of anything other than they bragged. I mean hell, plenty of people popped the cork and started celebrating in 2000 when the networks called Florida for Gore. That was all over something that wasn't true.
As for them having a missile system. That doesn't prove anything other than they had the capability to do it. Lots of parties in the area have that capability including ukraine.
Your evidence doesn't amount to any logic or factual error when someone questions it. It doesn't amount to proof either. But for what its worth, i think the rebels did it too. I'm just not willing to dismiss other possabilities based on what we are being told. You shouldn' either.