Can I dump Jello into a pan and boil it to get it to set?
Yes, you can boil the whole thing. You need to get the temperature high enough for the gelatine to activate. The reason jello reserves half the water is to cool it down faster and reduce set time, there's no issues adding all the water in the first place other than taking longer to cool down enough to set.

If you are in a hurry you can cool it down afterwards by putting it in an ice bath and stirring it for awhile.