Electromagnet Polarity Reversal Doesn't Matter?
If you want to demagnetize something, you actually have to use AC current, rather than opposite DC.  Assuming the problem you are experiencing is residual magnetism you should be able to test by sticking the magnet to other things.
Unless they've put a diode in the 2 lead electromagnet you're considering, you can reverse it, it'll just immediately remagnetize the other way and keep holding. Why do you assume the core is of an unsuitable material?