For (1), ask yourself this question: could you write a computer program that could compute this function? If so, then by the Church-Turing thesis, then you know that there must be a TM that could do the same computation, so the function is computable. If not, then you know that no TM could evaluate the function either.
For (2), remember that ambiguity is a property of grammars. There can be multiple grammars for the same language.
Hope this helps!