Have you checked whether the caliper works at all? A good indication would be if the disc surface is clean after a medium distance drive or heavy breaking from 80Mph+
If the caliper itself is not seized, then check the mechanism attached to it which operates the caliper when the handbrake is applied, there is a return/resistance pring there which may be knackered. The only way to check is to remove the wheel and look at the state of affairs, then apply the handbrake and see if the mechanism has moved.
Third thing you could do is bleed your system, any air trapped in that caliper will render it useless.
IMHO not a good idea to put WD40 on your cable, that stuff desolves rubber and eventually dries off with the potentional of seizing the cable. Dont do it, epsecially where rubber is concerned as it will crack it.
No comment on your heater, whomever fitted a new ECU for you, should be given the car back to get it sorted, its more likely something was left disconnected on re-assembly.
Good Luck, and let us know how you get on.
P.S. You know how it is, women do not tolerate another bella whilst they are around, two love affairs will get you in trouble in most cases. Try getting her some flowers when she is not expecting them