I can't get the water to divert fully from tub to shower and can't get the knob on the tub spout to pop up to divert the water flow to the shower.

If you can't break it free with a pair of pliers then the other option would be to replace it. They are inexpensive and can be purchased at any big box store. Simply take the old spout in and walk out with a new one. If you need help removing it click on back . Good luck, Tom