Yes offcourse dear friend you can continue ca final even after getting ATC certificate. I have ATC certificate and i gave my May 15 attempt, (i failed is a different story ;p).
Regarding working as tax consultant, ICAI should have clarified about the powers of ATC holder, but unfortunately ICAI is silent on this. but it is much helpful for those who could not complete CA finals and finding themselves at nowhere on the earth.
We can practice as tax consultant, although it is nowhere written by ICAI. But icai not even restricted to practice as tax consultant. But in society tax consultant is more popular thats why we have to use it. And ICAI will not say it illegal to work like tax consultant when holding ATC certificate.