How to change the texture of pudding?
Part of the problem could be the refreezing of the ice cream.  You know when you take out ice cream, eat at it for 5 or 10 minutes and then put it back in the freezer...and then the next time you take it out, some of the ice cream is a little icy and bit grainy?

I'm not sure you can solve this if you use the same recipe.  Instead, I would recommend using the highest fat chocolate milk you can find instead of using melted chocolate ice cream.

I really doubt the pudding didn't get dissolved completely.  If that really was the case, you could heat the mixture before you put it in the pie crust.