Pinks don't over analyse this!!
I do think you did your long runs too quickly. Could you go faster at a different marathon? Possibly but who knows a marathon is a different beast every time you do it and there are many variables that you may or may not be able to train for.
If you were very comfortable at your LSR pace then maybe you ran the marathon slower than you are capable of. But only you can answer the question Did you feel you could have done better on Sunday? None of us know how you felt then or now. The LSR pace for a 4:30 is 10.45ish. It could have been that you were pushing too hard on your LSRs which meant that you were "racing" them and not treating them as you should have.
But as I said only you know the answers to those questions.
You ran fantastically well and paced it brilliantly - getting a negative split is perfect. But do you think that if you had set off faster you would still have had enough in the tank to go faster in the second half?
You have learned a lot from this marathon and I think maybe you should revise your goals a little. However I think you need to get some more half mara speed before setting too high a target on yourself to give yourself some comfort room (or you need to do a lot more 20s in the build up).
Use the rest of this year to evaluate what your paces are. Try some really tough stuff and see where it takes you. BUT remember your back injuries of previous times and proceed with caution!! 