The firing pin stop was the first attempt to remedy the extractor from clocking. I had about 5 casings to the forehead out of a 50 round box. No jams, there is a brass trial on the front of the ejection port. One of the cases got mangled at the mouth. Shooting Star mags were used or this session.
The magazines are not causing your problem. My assumption is that you fit the firing pin stop so that the extractor does not rotate. The cases are still falling off the extractor. At this point my suggest is to put in the new extractor. You have to be concerned with bevels, deflection, and tension when fitting a new extractor.
The extractor can cause feeding problems or extraction/ejection problems if not properly fit.
Feeding
To allow thw the case cannot slide freely up under the extractor areas "D", "G", and "F" in the picture below must be gently rounded. Just a little bit of rounding should work. If area "D" extends too far up the tensioning wall "C" the case rim may not be in solid contact with the extractor as it moves down the tensioning wall during cycling.
The whole idea with rounding these areas is to allow the case to move into postion without encountering a sharp corner that could dig into it and prevent it from sliding into place.
A side benefit of rounding area "F" is that it allows the empty case to more easily rotate up as it encounters the extractor thus insuring ejection.
In the next picture you can see that the edge of the claw "B" does not touch the case anywhere. This is a perfectly fit extractor. Remove only enough material from "B" to just clear the case. Take off too much and you'll have extraction problems.
Deflection is how much the extractor is forced outward as the case rim slides into place. Ideally, deflection should be 0.010". Some will set deflection as low as 0.005" or as much as 0.015". The amount of deflection is contolled by how close to the firing pin hole the tensioning wall is when it is not holding onto a case. This is determined by the locating pad "E". The more material you remove from the locating pad the closer to the firing pin hole the tensioning wall will be.
With this small amount of deflection the extractor tension can be set quite high and it will function fine. Tension is simply bending the extractor so that it holds onto the case rim tightly enough to withstand the cycling forces without losing control of the case. Too much tension will impede the feeding process. Too little tension and you'll end up with the condition you're experiencing now. If you bend it too much, you can unbend it.
The front of the extractor should not contact the barrel.
Extraction/Ejection
If you can get the extractor beveled and fit as described above, you should be in good shape in terms of extraction/ejection.
As you've already learned a clocking extractor is not your friend. The firing pin stop should not allow the extractor to move and aft or to rotate.
So, these are the basics. There are other nuances and everyone seems to have their own favorite methods but if you can get the basics figured out, you should be able to solve your problem.