I'm assuming US gallons, and that the cider is normal ABV.
The yeast has dropped out of the cider?
Is your Campden sodium or potassium metabisulfite?
are the sorbate and Campden quantities correct?
FYI it's much more accurate to use a scale, with 0.1g resolution or better.
Sulfite antimicrobial activity depends on pH, so it's hard to say. You'd need to test the pH of the cider pomegranate blend to calculate the right amount. At pH 3.5 you'd want around 0.81g potassium metabisulfite in 4.5 gallons, about 1/8tsp. (Assuming 1 tsp = 6.6g)
2.5 tsp potassium sorbate in 4.5 gallons gives roughly 360 ppm sorbic acid, which is too much. 2 tsp should be OK. (Assuming 1 tsp = 3.3g)
Depending on how much pomegranate juice you add, these numbers would increase because it's the final volume that matters.
do I need to bring the temperature of the cider back up to 68° before adding them
Yes, potassium sorbate is not very soluble at cold temperature.
how long should I wait before kegging to let the sulfur off-gas?
You don't want it to off-gas if the amount added was correct. You could rack directly into the keg along with the sorbate and sulfite.
You want around 0.4-0.7ppm molecular SO2 present to prevent bacteria from creating off-flavors, and not have the sulfite cause an off flavor itself.
.... If you can't test pH, you could just aim high and if there's an detectable flavor from the sulfite (like burnt match) you can take action to help off-gas some of it.
Hope this helps