How bright can white dwarf stars glow as they accrete matter?
There might be a problem with your calculation, because by the time a white dwarf made of helium gets up to 0.5 solar masses, not only are the surface layers hot enough to fuse hydrogen, they are actually hot enough to fuse helium!  That's what will happen to the Sun when it ceases to be a red giant.  You are talking about carbon white dwarfs, but they are similar to helium white dwarfs.  So hydrogen fusion at the surface of a white dwarf is a common result-- it is what causes a phenomenon called a "classical nova."  So you are on the right track-- but what you are talking about is a well-known area of study.