|Answer 1||Lucy's the only Celestial Spirit Mage that's not dead or in prison; however, just because you have more Celestial Spirit Keys doesn't mean you're a better Mage; thus, Lucy isn't necessarily the strongest.|
In the GMG arc another Celestial Spirit Mage by the name of Yukino Aguira was introduced, so the part about Lucy possibly being the last free and living Celestial Spirit Mage is no longer the case.
|Answer 2||As the previous answer states, the number of keys does not determin the strength of a Celestial Spirit Wizard. Angel was able to overpower Lucy quite easily despite having fewer golden keys than Lucy. If it hadn't been for Hibiki's intervention, Angel would have certainly killed Lucy.|