1. It's okay to check your partner's phone; why treat it as a bad thing? Many relationships could be saved if partners were to embrace this.
No, it isn't. It's being treated as a bad thing because it is.
To preface, I've been in my current relationship for 20 years with my now wife. My wife and I have unfettered access to one another's. I know the pass code to hers, and she knows mine. We regularly use each other's phone to look up something, watch a video, queue up a podcast, and so on.
Before my wife, I have been hurt and betrayed a time or two. I have never demanded access to my wife's phone, laptop, or anything.
Never have either of us done an investigation of the other's phone. And neither of us have required access as a foundation of trust because that's not how trust works.
You are confusing required access with access freely given. Required access is oppressive, fascist, and explicitly states you distrust the partner in the relationship. If there's distrust in the partner, then there's no point in being in a relationship with that person.
There are no high-minded, intellectual counterarguments. Only devolved cavemen would demand to go through everything on the phone to force a relationship to work.
3. Doing that to a kid — the writer is trying very hard to portray this guy as a villain.
I assure you, as a parent, people like this exist. The writer doesn't have to try.
Although, the actual physical harm is usually due to being on illegal drugs. So, yeah, that's a stretch.
Only explanation that works. Maybe the guy family is more powerful, or maybe he knows she is into Arata, but on that case, he would be less worried about her "cheating" with random guys and would worry about Arata himself.
In any case, even for the older sister wanting to avoid trouble for her family, I would be mad at Banri for possibly let things get so badly when she could have just get some help.
She even let her sisters talk about the guy as if he were a saint. Would she even let that monster meet her sisters?
Most abusive relationships start off as picture perfect. Then, after the honeymoon phase is over, the abuse begins in small, tiny ways as the abuser starts pushing that boundary until it becomes overwhelming like we see here. Once the victim finds themselves there, they find it hard to leave and will even justify the abuse because they're ashamed they became a victim. They'll even make the abuser sound like a good person.
There are a bunch of reasons victims stay, and most have nothing to do with blackmail or family status.
https://www.joinonelove.org/learn/why_leaving_abuse_is_hard/