Ouch the neck has got to be the worst possible place to pinch a nerve >< it would hurt to do pretty much anything,
As for anal sex, I can totally relate to the accidental and painful experience that you described because I've had the same thing happen to me... "I thought that I was going to die!" Luckily, I've never experienced the additional back pain. But it doesn't surprise me that it could or would happen... because I suspect that you automatically tensed up your back muscles when you were unexpectedly penetrated. And, that tensing up most likely aggravated your pre-existing back problems. As far as other repercussions or consequences, there's the unhygienic aspect with the risk for bacterial infections caused by the penis entering the anal cavity and into the vagina. But, if you're talking specifically about other back problems, I suppose that a sudden strain or movement could cause a pinched nerve. I got a pinched nerve in my neck many years ago, when I was giving my bf oral sex.