Which name should I choose for private abstract functionality with a public proxy?
I think that it's more a matter of taste. If I were to come along behind you, I wouldn't have any particular problem with fEval and fSimplify.

I'm not sure that I would like concreteEval.

In my own code I tend to use doEval, but I could also imagine using evalImpl or myEval, depending on what it is exactly that is being done by these functions.