Is there a difference between type signatures a -> b -> a and c -> a -> c?

Nope. Think of a -> b -> a as This Type -> Another Type -> This Type. Then both of them fit given pattern. However, something like a -> b -> c wont fit: This Type -> That Type -> Yet Another Type is (generally) different; except the edge case when c = a, which yields the pattern you're interested in.

There is no difference. Since a, b and c start with a lowercase, these are variables. You can rename variables, and this remains the same, as long as two (or more) variables do not "clash".

Such clashes can happen if you rename a variable such that it has the same name as another variable, or when you rename two (or more) variables to the same new name. If you would rename the variable a for example to b in your first code fragment, then we would get b -> b -> b, but that is not the same, since then we enforce that the first and second parameter are of the same type. Whereas in your type signature, we have the freedom to pick two types that can be the same, but that is not necessary.