Is it possible to just have a garden variety headache or neck ache not related to your PCS, and how do you tell? And, does one cause/interfere with another?

I'm not a MD but I'm sure it's possible; I would consider whether/how often the "garden variety headaches" occured before the injury, and if the severity and location of the aches are similar to they way they presented prior to the injury.

Last few months I've had a couple of illnesses which mimicked some of my PCS symptoms. They were pretty scary cos at times it felt like a major relapse. Of course it's possible to get normal headaches or other illnesses when you have PCS, it's hardly going to make you immune to them is it?

You just have to take things as they come and let it happen. If you have a 'normal' headache you should be resting and drinking a lot of fluids, which is the same as you should be doing if it is a PCS headache, so as long as you don't get too stressed about it it doesn't really matter.