Most people do not accept that most Guantánamo detainees have been/are being held without ever being charged or tried. The terrorist in this case is the US as harsh as that sounds.
People are right not to accept that because it isn't true. Of the 35 Guantanamo Bay detainees, 3 are being held without charge. https://www.closeguantanamo.org/Prisoners
Simply? The goals is to have a prison outside of the US where the US (i think mainly CIA) can do things that it cannot when in the US. The goal is clearly to subvert the "rules based order" they so vocally claim to uphold.
No it's not. As the person you responded to pointed out it's basically torture when you're preventing suicide. If someone is such a permanent problem, execution and suicide are obviously options, but there are also many prisons that provide the isolation you want without being as bad. There's no excuse and it's far from "needed."