SOMETIMES you can loosen the bond between heatsink and CPU when it is re-installed in the CPU socket and the locking lever locked down. But do NOT fasten the heatsink to the support brackets. This provides a reasonable fastening of the CPU distributed over all its pins. Then carefully try to twist the heatsink while the CPU chip remains held firmly by the socket. If you can get the heatsink to twist both ways a little, then its bond is reduced substantially. You MAY be able to lift it off then, but don't pull too hard. If not, un-latch the lock and remove both (CPU and heatsink) from the socket, then try to separate them again with a knife or something.
If you cannot twist it, plug the CPU fan in again and turn on the system for normal operation, but still do NOT fasten down the heatsink to its brackets. You have no need to ensure tight heat contact with the CPU - it is already stuck on! Now try to do some real work so that the CPU heats up, but not so high as to be overheated. This may do more heating of the old thermal paste that you were able to do with the hairdryer, and soften the paste enough. When it's good and warm, shut down and try again to twist the heatsink and loosen it.