>>13358247That is not the question, we already know any valid puzzle can be solved with guessing and backtracking. The OP's question, which I share, is that for a valid puzzle is it always possible to prove that at least ONE square MUST be a specific number. Whether humans can do this or not is irrelevant because, given the correct algorithm, a computer could do it.
>...there is nothing significant differentiating forward planning and back tracking.Can you explain what you mean by this? If I am interpreting this correctly and this has been proved then the answer to the OPs question is yes.
