Yes, it would cost much more to consult with a lawyer than to apply for a citizenship certificate, and what would be the benefit? Let's assume that a lawyer thinks the OP has a good case for citizenship by descent. Would the lawyer just ring up CIC and have them issue her a citizenship certificate? No. She would have to apply for a certificate just the same. So, why not do it first? Then, if she were refused (and I think she would be), she would only be out $75. At that point, she could decide whether to consult with a lawyer, and, if the lawyer thinks she has a good case, contest CIC's decision in court (costing many thousands).
This particular case is not that unusual: a person born in the second generation seeks citizenship, but it is precluded by the restriction (to 1st generation) imposed by the 2009 law. Your opinion above ("Doubtful...") is correct. All that you forgot is Bill-C24, which in 2015 gave citizenship to persons born in Canada who did not become citizens in 1947 (the OP's grandfather) and their children in the 1st generation (the OP's mother). All CIC can do is apply the law as written, and the law did not allow an alien (non-British subject) to become a Canadian citizen in 1947. It also would not have allowed the OP's birth to be registered in the 1960s (remember...that was a requirement before 1977) even if her mother WAS a Canadian citizen, because mothers could not pass citizenship on to their children (born in wedlock). Therefore, the OP could not have been a Canadian citizen before the 2009 restriction on citizenship by descent went into effect. There are just too many impediments to get over for the OP to claim Canadian citizenship.