<p>After a brownout a print failed and it got the whole hotend covered in PLA. I am now in the process of replacing some parts on the hotend (one of the thermistor legs broke of) and also wanted to take the heater cartridge out. The problem is that the bolt that locks the heater cartridge is stuck and I am now afraid to strip the head.</p>

<p>Is there a trick to remove the bolt with the smallest probability of stripping the head? Is it better to apply heat or not? Should I soak it in Acetone or some other solvent? etc.</p>

Acetone will not dissolve PLA, there are other nasty solvents that do that, but it is not recommended to go that way.
You could apply 12 V (if the cartridge is 12 V that is) directly. As the thermistor does not work, you cannot use the printer board. You should take caution not to heat it too far or too hot, but from your experience with your own machine you should know approximately how long it takes to heat up the hotend to working temperature. E.g. start with a minute and try to pry off the lump of plastic with pliers. If you can reach the bolt/worm screw, try heating until you are able to insert the hex key or screw driver.
This video
from Josef Prusa shows exactly what you need to do what is described above. The only difference is that you need to connect the heater element directly to the power supply as your thermistor is broken. So don't leave it on too long or you will bake the filament to carbonization, which is much harder to remove. Try to cycle it on/off.