mine did the same and BMW Scanner 1.4 identified that it was the internal motion sensor that was activating so i just disabled it and its been fine ever since.
More likely to be a coil pack than the plug - I've never known a plug to fail in my fifty years of tinkering with cars. Oil contamination or impact damage yes, but failing on its own - no.
I used the wrong rear springs simply because that's all that was available quickly used when i needed them for its MOT. I have no plans to change them.