I wouldn't dump hardener in there, it won't mix properly and you'll just have more goop. Also, simply epoxying around the cord will not fix the issue in the cap, which is that the epoxy hasn't properly cured.
I would recommend starting over. You could try to pull it all apart and clean the element base, but any solvent that will sufficiently clean the partly cured epoxy will most likely compromise the integrity of the plastic postion of the element. If it were my system I'd chalk it up to a learning experience, get a new element and cap, clip the existing cord as close to the cap as possible and do it over. Next time mix the epoxy in a separate container and pour it into the cap.
The power to tax, once conceded, has no limits; it contains until it destroys. I was not joking when I told them to dig into their own pouches. It may not be possible to do away with government sometimes I think that government is an inescapable disease of human beings. But it may be possible to keep it small and starved and inoffensive and can you think of a better way than by requiring the governors themselves to pay the costs of their antisocial hobby?
R. A. Heinlein