I would say that if the joint has failed at the heel it will be suspect over the whole of the joint.
When you heat gun the mast to remove the varnish, you may find that the glue will fail some more and open.
I would put a light wedge in the split and see if it goes on its own while your heat gunning.
In days of old when we did not have Super Epoxy glues, the mast was varnished in the groove before glueing together making sure that no varnish was on the glueing surface.
A way to varnish the inside of a mast is to run a string down the groove and tie a small rag with varnish on it, then pull it down the mast and Hope!!
You can glue the mast together again with Epoxy as it is a glue as well as a coating resin, just use the right hardener with the resin which I think is called, Coating Hardener, to get a varnish like finish.
Dont forget to clamp the mast to a straight edge of some sort so keep it straight when gluing.