Dear Neville,
A mumar l’chol HaTorah is one leaves the Jewish Community to live among with Akum for his own benefit. It doesn’t matter what he does or doesn’t keep. Nor does it matter what he believes in regard to Hasem, His Torah, or our purpose on Earth. That would be a min, kofer, or apikores. Just leaving makes one a mumar. It is that simple. There is need for an inter-faith roundtable to figure out why we are not Christians. It has it’s applications in kiruv et cetera, but it doesn’t matter among Torah Jews. Which is where this Chabad debate takes place. I really don’t know what you want out of this part of the argument.