Is the pinched nerve a chronic condition? If it is just an occasional thing with no rhyme or reason for the cause other than circumstantial, then I'd be suspect of the advise to sit up more. That will put more weight on the butt which will feel worse toward the end of long rides.
If it is a chronic condition, the maybe it's time to consider a recumbent which will allow for more body contact points to give more support that you can't get on other type bikes. I don't ride them, but I can see there may be a day when I'll have to.