They were painful but still not as painful as it was for Cordy especially the more use to them he got. With Cordy, the only way for it to not be painful for her was when she agreed to become part demon. None of those storylines would happen for Doyle....

I am not sure... without the backstage drama, I do feel Doyle would've lasted in the long term. Spike was suppose to be killed off as you mentioned but his popular kept him around. With Doyle, he was quickly fan favorite with his first 9 episodes so if Glenn doesn't have the backstage thing going on, I don't see why he would get killed off.