I thought that was just to bring his internal temperature up to get Luke warm.
Nah, it’s sci-fi healing fluid.
You could do the other thing with a blanket. Or a Tauntaun.
Correct, bacta tanks are also used on Vader who was damn near incinerated. It being body temperature has something to do with it though because your body wouldn’t have to work as hard to keep your core temp stable.
I’m not sure there is any temperature that would both allow the body to remain heated exclusively through passive heat generation while also letting the body avoid overheating. Granted, higher thermal conductivity of liquids can make the task a little easier, but still I’m not sure there exists such a perfect temp.