This is an excellent, excellent response and thank you so much for it!
House Sorting based one what the character values is a really popular way to look at things, and I agree with it, but I do think that you have to consider the way they go about protecting what they value, too. I think that both Gryffindors and Hufflepuffs are highly family/friends-motivated, they just differ in the way they express this motivation. Hufflepuffs want to be there for their friends (e.g. give them support and hugs, make them tea, band together in general to deal with problems), but Gryffindors want to defend theirs.
(I hear what you're saying about how Jack steps in and is brave for his friends, but look at Harry himself. Harry couldn’t care less about the glory and fame either; all he really wants is his parents back and his friends to be safe and to go and live a quiet life where nobody will talk to him about being the Chosen One again. But he’s called to be his friends’ hero, not their support group, and that he’s able and willing to rise to the call [as well as having a bad case of daredeviltry] is why he’s a Gryffindor.)
You make a really good point (like, a super-duper good point) about how Jack ditches all defiance when he turns scab and that that’s really uncharacteristic of a Gryffindor, but rather than this being a natural course of action for him, I actually see it as being a huge character development moment, because Jack’s already lost one friend to the Refuge this movie already, and being defiant (let’s soak them for Crutchy!) did jack-all (excuse the pun :p) to protect him. So Jack’s trialing another approach now—one that comes less naturally to him.
Jack: I don’t understand either, but just get out of here!
Right there, I think Jack is really struggling with the decision he just made to roll over and give up, because prior to this he would absolutely have pushed back and he still really, really wants to.
In the end, though, I think it’s Jack’s natural leadership that pushes me right over at Gryffindor, because I really don't think any other House leads like a Gryffindor leads.
(But thanks again heaps and heaps for your response, I understand so much better where the Hufflepuff!Jack people are coming from now, and I have got to say that if we don’t act together then we’re nothing, if we don’t stick together, we’re nothing; and if we can’t even trust each other, then we’re nothing! is an exceedingly Hufflepuffy creed. So…he could go both ways equally easily? :) And also, okay, but a Slytherin and Hufflepuff dream team, all right, you’ve sold me there, that’s miles better than Slytherin and Gryffindor. :D)