I don't see the problem. Extended detention and repeated interrogation is normal around the world. Visit Rikers island and look at how long people are held there prior to trial (years). The focus on confessions isn't a Japanese thing. Look at how many americans/canadians/brits/auzzis agree plea deals, a form of confession. Repeated questioning isn't torture. In the USA it is true that police have to stop asking quest…
My unpopular opinion: Questioning someone longer than they want to provide answers, when that same person is unable to leave the interview is a form of torture -- change my mind.
The Merriam-Webster definition above is weak.
One discussion around torture with my philosophy professor (thankfully the french engineering cursus has one or two interesting classes) really stuck to my mind. We were discussing a text where a french general that had practiced torture in Algerie. He said something that amounts to "I have asked my soldiers to electrocute me so I could see how it feels like, it is not that bad so I felt ok about using it on algerians". It misses the whole point of torture : that you totally lose control, which is where the agony comes from. Anecdotally, I have also been briefly electrocuted by accident once. The pain was indeed not that bad.
But I have not been tied to a chair and subjected to somebody electrocuting me for as long as they like.
This kind of repeated pain is what is impossible to resist to for very long. Not to mention that any sensation can be used that way, tickling is also a form of torture.
So I can see why you can construct that forcing somebody to answer questions, possible very hard ones about somebody close to them that just died, can also be a form of torture.