I actually have to agree about Alexander being Graham's biggest "failure". To those of us playing the games it might not be, but try thinking of it from a parent's perspective. As Alexander's father, you'd think that being unable to save his son while having done so many other great things (even before the games he was Daventry's best knight) would be seen - to him, at least - a failure.
I don't see how considering it to be a failure is constructive. If I were a parent, and I lost my son, and tried as I might, I couldn't find him. I think I would be showing more care and be grateful that he arrived home okay. I would like to think, "he's finally home", rather then "this is going to hurt my perfect adventuring record". The only reason I find the word "failure" to be an inaccurate metaphor for Graham's and Alexander's relationship is cause I can't detect any animosity between them.
Alexander grew up under the tutelage of a dark wizard that was going to eventually sever him from the living world but was willing to let him live so as to finish the chores in the house. Alexander being the heir to the kingdom of Daventry is comparable to winning a lottery ticket. From where Alexander was standing (over Manannan's kitty-self), he could have everything he wanted, a family, a home, a real father, the glamorous lifestyle, etc. I don't see how Alexander would hold a grudge if he wanted to fit in with these people.
Also, Graham was in a very dark time with the kingdom of Daventry being in the wreck that it was. Would Graham truly feel guilty over something that couldn't be stopped almost 2 decades ago? As Alexander comes into Castle Daventry with Rosella at his side and the dragon's three heads dangling in his arms, would Graham ponder, "He is my only failure", or would he think "This is my son!". So Alexander would get the family and home he always wanted, and Graham gets the son to pass his adventurer's cap on to. Saying Alexander was Graham's "failure" is so pessimistic, I'd rather think of Alexander being a "missing piece" to Graham's metaphorical puzzle (which is his complicated life).