No, "should" is not generally interchangeable with "would". The easiest rule of thumb is that when a speaker uses "should" she is prepared to explain why, and when a speakers uses "would" she is prepared to explain why not. Here are some examples:
Muskie should have won by a huge margin [because he is much better than the competition].
Muskie should win tomorrow [because he is much better than the competition].
You should have heard by now that I'm OK [because I saw the doctor talking to you].
You really should have started that paper more than 8 hours before it was due [because it's very difficult to write a quality paper in less than 8 hours].
With "should", the speaker always has a reason why something did/will occur. If the speaker's reason [in brackets] isn't explicitly stated, it is still implied. Now consider "would":
Muskie would have won by a huge margin [if he didn't blow his engine].
You would have heard by now that I'm OK [but you didn't buy me a cell phone, so I couldn't call you].
I would have started that paper earlier [if I wasn't so busy with all my other homework].
With "would", the speaker always has a reason why not - why something did not occur. The speaker is more likely to explicitly state her reason [in brackets] with "would". If we are using a future tense, the why not rule of thumb becomes awkward but still workable:
I think she would like this as a gift [if we decide to buy it for her].
In sentences like this, everyone understands that it is hypothetical, so the speaker would rarely actually say the reason in brackets [if this hypothetical situation ever arose]. (See what I did there!)
One final note, which makes this kind of tricky, is that you can still use "should" even when you state a why not reason as long as a why reason is still stated or implied:
Muskie should have won if he didn't blow his engine.
Actually means:
Muskie should have won if he didn't blow his engine [because he is much better than the competition].
So in sentences like this, "would" and "should" are almost interchangeable. Just remember that "should" means the speaker knows why and "would" means the speaker knows why not. If the speaker happens to know both, then she can choose "should" or "would" almost equivalently.
The adverb ever is mainly used in negative and interrogative sentences to mean "at any time". Besides, it is used for emphasizing that something has never happened before or should never happen.
In the first sentence, the last ever concert means that it's the last concert that the archestra is going to perform; it will not perform again in the future.
In the second sentence, "the first ever fall in profits" means that it's the first time that there has been a fall in profits; there has never been a fall in profits before.
Best Answer
There's usually no significant allusion to "unavoidable misfortune" with the cited construction. Compare, for example,...
The only reason you don't normally hear...
TL;DR: Including had in the cited context usually makes no difference to the meaning. But it's idiomatically well-established for the context, and at I think at least some people (besides myself) would agree that if it is included, there's at least a slight implication that whatever happened, was heard or seen, etc. was an unlikely / unexpected / surprising / unfortunate accident.