I think the issue with Boruto is that when you compare him with Sarada, he does come off as being immature. I mean, his problem is that his dad is busy being Hokage and doesn't have anytime to come and play with his son (or from Boruto's perspective, put his family first ahead of the village). Boruto doesn't understand the responsibility that comes with Naruto's job that makes him unable to be around, and that in turn makes him act out, and since we don't see much from his perspective he comes across like a spoiled brat.
Sarada on the other hand, despite being the same age and not being as mature, is actually having a legitimate crisis in that she has no clue who she is in the world. She wants just to learn who she is, who her father is, why is he away what's he like etc. but instead of there being a reasonable, apparent explanation like with Boruto, she is meet with non-answers from everyone close to her to the point she has no idea why are people hiding the truth regarding her birth. And when she faces that, she decided instead of acting out to go to ask other people who might tell her (so she being more reasonable than a lot of the kids we seen so far, outside of maybe Inojin).
So it's easy to see Sarada as being more interesting than Boruto, because not only do we get heavily invested in seeing Sarada's perspective and inner thoughts, we also get to see her try and actively find out a solution to her problem. Compared with Boruto, who we only get to learn about from seeing little snippets whenever he runs into Sarada, and has been doing nothing (from what we see) except bout or pull pranks, it's easy to see him as not as interesting.
I think we might be able to empathise with Boruto more if we see more from his perspective (I mean, I do get where he is coming from, since I had a father I loved and wanted to be with as long as possible, and as a kid didn't understand why I couldn't spend all time with him) and see as he comes to terms with his dad not being around as much as when he was a kid.
But I guess that is what the movie is for.
As for Naruto and Sakura being interesting characters ... Naruto is for sure (even if the core of his character and its arc is simplistic and traditional, he has 700 chapters to flesh him out) but Sakura was hardly ever interesting to start with, and he development in the main series had been minimal to nonexistent towards the end. Making more fleshed out characters than her isn't that difficult.
Affinity has to be by inheritance otherwise it wouldn't stay so consistent for each clan. And while I'm sure Kishimoto won't ever go into scientific detail about the genetics at this point, it wouldn't be a stretch to assume that while personal and clan affinities can be inherited, trained affinities can't. Because it would make inheritance a moot point if you could train your genetics like that.
Unless the clans had some Sage of Six Paths mojo that allows for anyone of them to switch or gain the required affinities for their clan.
Then again, even if trained affinities can't be passed on, that might be a moot point with Naruto and Sasuke since they didn't exactly train to gain their new affinities, so much as awakened their inherent Demi-God gene that their past lives always had or something.
But that still works for Bolt since Hinata's personal affinity is either Lightning or Fire. And since Hyuga's seem to have an affinity for fire, you could see Hinata being in the same position Sasuke was: their personal affinity is lightning and then they get fire as a bonus thanks to their clan. Hence Bolt the lightning user.
I suspect that would be the justification for it, yes.
Unless it turns out Naruto Demi-God gene overrode that and Boruto has all affinities, but decided to learn Lightning first because Sasuke was his master.
