If one has a root canal which has been started in August last year and was finally filled with Guterpercha etc in November:
is the damage already done?

The damage was done when they attempted to clean all the root out of the tooth which is next to impossible. This caused the tooth to be dead because there is no blood going into the tooth to fight any infections. The tooth had some kind of infection that caused you to get the RC, so it's not to hard to to realize that some could still be in the tooth.

Assuming the "heat" problem was from the RC might be an indication that all is not right. As far as how long?  No way to tell.

Not worth the worry when the solution is so simple.