Conclusion: In all my cases I succeeded in opening the stuck shutter, BUT: often stuck shutter had a reason. Might be increased friction or some debris, dust, etc. So even if opening shutter was successfull, it could happen that it shortly thereafter sticks again or even remains open.

So even if you succeeded in opening, reverse polarity, close it again, open again,... Do this with decreasing voltage down to less than 4volts. Only if this works flawlessly to 100% again and again you have good chances really having solved the problem. Make some breaks between to give the coil the chance to cool down and afterwards keep trying!

So good luck! Hope this will help, though these cameras are rather old now. It surely also applies more or less (at least in principle) on others of the Powershot family.