......Pakistan has not MESSED up the Kashmir issue, ...
Well that's your opinion and it should be respected no matter how incorrect it is.
...had Pakistan not taken any action in 1948 they'd control all of it, and we'd be left with a dry infertile chunk of a country.
You are correct that Pakistan did support the uprising of Kashmiris and later sent in guerillas.
However Pak-controlled Kashmir does not control much of the water sources compared to Indian controlled Kashmir.