Hey Mia,
I have been gone all week for family stuff myself and am just reading this entire thread.
My heart goes out to you. I am so very happy that you got to see your "Pop" and say what you wanted to him. I personally believe that people in a state such as you described DO hear us, they just can't reply.
Please don't be too harsh on yourself for not getting back to him on your way out of town. He knows that you came to see him and that is what's important.
As for grandma, it sounds like they were extremely close through their lives and, yes, sometimes one follows the other in passing. (Mine did 6 weeks apart.) Tell grandma now anything that is in your heart. And hopefully she will be around to remember it and live with the knowledge for many many years. No reason to wait to tell her.
I am so very sorry for your loss and please allow yourself to be sad. It's important for your own growth and mental / emotional health. Hugs and warm thoughts. And remember, he is still with you so long as you remember him and share his special place in your life with others.
Here if you need anyone,
Disney Dreams / Jegrezo