Yes. I've been using five pound weights (and even some 2.5lb ones for certain movements) and I've definitely seen my definition and mass improve. If five pounds is easy, up the weight a tiny bit to continue seeing progress.
No. And i will explain.
A 5 pound dumbell wont be enough to test the big muscle groups. For those your own bodyweight would be a lot better resistance weight than a 5 pound dumble. And i believe the big musclegroups are the one you should focus on. A 5 pound dumble can probably be used to build muscle but it should be one of the tools in your arsenal.
I was told a woman shouldn't lift to much weights. I wouldn't want to be a big muscular woman. We just want definition. And a five pund weight is perfect for that.