You can finish or not finish the inside, it is your customer that will either approve or not approve of what you have done. I started making calls in the early 1960's, I finish the insides of my calls, I have never had a complaint.
As to the CA, on the rare occasions that I use it, I use a 3/16" aluminum mandrel that was split to hold a piece of paper towel, and apply CA to it and then to the inside of the barrel. First I put on 2 coats of an oil finish, sand lightly after the second coat and the apply the CA.
Marvin