It seems that you are not suitable for injections - in most areas, once you have had one trip to A&E with priapism, they will no longer prescribe injections for you. (I am assuming you did go to A&E when you had a 4 hour erection?) However, worth asking if you can try Invicorp before you write it off as a lost cause.
I wonder if you might have more success with the pump if you had some coaching from someone. It is a knack to get right but if you are able to get engorged in the tube, the problem is technique rather than mechanical. Many men just can't get on with the pump but some do very well with it - we used to have a member here, Adrian, who only used the pump and was a godsend at helping others to get their heads round it - he helped me & OH enormously in the early days.
Based on your response to injections, I am also wondering whether your non nerve-sparing RP actually was totally non nerve-sparing? It may be worth trying viagra or levitra tablets just to see what happens. But if you do that, do ensure that you take it on an empty stomach and at least an hour before you need it!