"Is it not difficult some 60 years later to offer logical, fact based opinions on what was and what wasn't?"

Probably easier in fact 60 years after than it would have been at the time. And the passage of time in no way makes the issues irrelevant.

And the central issue as I see it is this: if Hiroshima can be accepted as a justifiable and moral act, on the grounds that on balance it caused less suffering than it prevented, that means that, on these same grounds, it would be possible to justify absolutely anything.

It may well be true that most people do believe that - but if that is so, they should accept that there is no fundamental moral gulf between them and those who would justify terrorism on any scale. Just a disagreement about a kind of arithmetical evaluation of consequences.