No, it's a weird story, judging by what I've gathered from Wikipedia... the seppuku happened after what has been called a "failed coup", though that seems like a somewhat exaggerated description of what was merely an attempt to take over an army base (or something like that). When it failed, Mishima committed seppuku and one of his companions had the questionable honour (by our standards, that is) of finishing the job by cutting off his head. But supposedly Mishima had been planning that seppuku for a good while.

In any case, it's not so much the book that he considered a failure as his own life, I believe. Still according to Wikipedia, the final book can easily be read as a very negative judgement of his own life.

Oh yes. Here too there are anglophiles, and Honda's philosophizing about natural law is mostly based on Western ideas, going back to Aristoteles and all. But as I said, the gap is greater - Turgenev's anglophiles are far closer to actual Britons in mentality, whereas Mishima's are still quite Japanese. It's also mentioned a number of times in terms of architecture - the Matsugaes have a western-style house as well as a traditional Japanese one in their park, and spend quite a bit of time in the former.

And of course, since this is set fifty years later than Turgenev's book, there are some new Western things that didn't exist yet in Fathers and Sons: movies, cars, grammophone records.

Yeah... the problem is also that his immaturity and capriciousness is largely responsible for most of the drama/tragedy that follows. It's not really genuinely tragic if the blame is so obviously his own, even if he then tries to make up for it.