[everybodyknows]

Not a chemist here, so I'm wondering which is the higher energy state: fructose+glucose dissociated (into HFCS), or bound together into sucrose (white table sugar).

https://en.m.wikipedia.org/wiki/Sucrose

Why doesn't either HFCS want to crystallize into sucrose, or sucrose want to melt into 50% HFCS, given a bit of time, water, heat, or stirring?

[vel0city]

I realize I never really answered your other question. Its tricky sticking the two sugars back together into sucrose, as heating it will generally carmelize it. After all, when you carmelize sucrose you're breaking that bond and oxidizing the glucose and fructose, so heating HFCS will ultimately yield the same thing in an oxygen environment. You'll generally need some kind of chemical process to link it together, it takes a bit of extra energy to join the two sugars back together.

Sucrose will hydrolize and break apart in water on its own, but it does take some time. Adding a little bit of acid will rapidly break it apart.

[vel0city]

I don't doubt there's a lot of metabolic differences between sucrose and HFCS. Given they're free instead of bound I imagine it hits the body differently, and some studies do suggest this. Gastric acidity easily breaks the bond though, it rapidly converts to glucose and fructose in the gut. So I imagine most changes would be with our body's early detection in the eating process, such as the taste buds and other senses like that.