Tathanen, tt seems like you're bending over backwards to make it consistent (which is something I do all the time too, admittedly)
I'm typically frustrated by Time Travel movies that don't make sense, and am a huge fan of those that do. So yeah, I make an effort when I can. But really, in this case, I'm just describing what I see.
The problem with this otherwise reasonable theory (time travel causation paradoxes only affect the individual time traveler, not other aspects of the timeline) is that it means the kid will still probably grow up into the Rainmaker.
I wouldn't say that at all. Obviously the whole future is changing, since someone time traveled into it. The second the timeline is split by backwards time travel, the future is totally unset, and can be anything.
The key is that Bruce traveled
into a timeline. He is the only foreign element. The only paradoxes that could conceivably arrive are those that involve the nature of his existence. Traditionally you'll see a time travel story like this where the traveler goes to another timeline, and is isolated from it, he's in a temporal bubble of immunity since it's not actually his timeline. This movie though (and Back to the Future to a point) suggests that the time traveler manages to bond to the new timeline in some capacity. Thus the scars, the memories, disappearing at the end.
Really there are only two kinds of time travel that I think are air tight. 1) Split timelines where the visitor is unaffected by what he does to the timeline, and 2) single timelines where you can't change the past because if you travel into it, you were already there the first time, and probably caused the thing you're trying to prevent in the first place (Lost, 12 Monkeys). Looper can't be the second one there, since we see that the past is absolutely being changed by Bruce traveling to it (we see his version of those events as he lives through them, pre-travel). So we're left to wrangle with option one.
Back to the future has always bugged me, since it seemed to belong to the first type there, but then you have shit like Marty fading. Looper seems to have the same problem, where the time traveler is affected by the changes he makes in a distinct timeline. It's far more pronounced in this film, though, with "ripples" of his changes affecting him throughout the process.
The only way I can make either movie make sense is by assuming a time traveler visiting a new timeline is still affected by it somehow. That he finds himself halfway between timelines, hailing from one, but still governed by the new one somehow. It's only him, though. If, in Back to the Future, Marty's parents hadn't gotten together and he faded away, everyone in the past there would have still remembered him, and been affected by what he did. (That's the way I see it, at least.)
To me you can't just "try really hard to avoid creating a paradox" by the actions you take in a time travel movie, as you suggest that maybe Jeff Daniels did. Rather, the time travel rules need to make paradoxes completely impossible. If you have to twist shit around to make that feasible, so be it, but a paradox is just that: a paradox. I won't allow them!!!