I agree that the movie did not do a good job in actually establishing the political state of the galaxy. It makes a fuckton more sense after getting information from the novelization and some of the new canon stuff.
Basically, what Luke, Han, and Leia did at ROTJ was not pointless; their actions pretty much started a chain reaction that pretty much ruined the Empire in the next year. The kind of government they fought for essentially was a peaceful democracy that didn't see a major war for 30 years. The First Order is literally a fraction of the remaining Empire that didn't want to either leave that military life or merge with the Republic. Instead, they took their ball and went home so to speak by retreating to the Outer Rim. Sure, they slowly built up their forces from the Empire's collapse up to the point of TFA, and some planets/governments/crime rings secretly supported the First Order on the side. But the Republic didn't take them seriously and thought the First Order was just a joke of a fringe group.
Of course, Leia knew what was up (either this is how she figured out about Snoke in the first place or I dunno) and the Republic was like "eh, whatever, take these X-Wings and do your shit with a new Rebellion since you seem to be good at that."
But other material also established that the Republic and the Senate rotate between home worlds, so who knows what kind of blow the First Order made with the Starkiller Base (if they even made a major dent at all, militarily or moral-wise). But after finding out the above info, the world of TFA did make more sense to be. But again, I also agree that it could have been handled in the actual movie a lot better than it was (Where people didn't even know if the planet that got blown the fuck up was Corusant or not).
Now the character motivations are one thing. Han's change makes sense given his character. I cannot really extrapolate a lot from Luke's situation given that he had no lines. In my head canon at the moment, for what little it's worth, I believe that after the Kylo Ren fuck-up, he's trying to find the First Jedi Temple simply because he's cognizant that he's missing something. I mean, the last Jedi Order utterly failed after one turned to the dark side, and now Kylo Ren threatens to do the same. Something about this Jedi/Sith duality is not working and he might be trying to go to the source to figure out why there's a Jedi order at all, let alone if there is something about the myth/teachings that just got lost over the generations.