Quote Originally Posted by AlexAK View Post
2) Will have + past participle for a past situation:
"Most people won't have seen last night's lunar eclipse" (rather than ...wouldn't have seen...?). Why not "...might/may not have seen..."?
In your example, you can't replace will have +p.p with would have +p.p, because the first is the future tense - something will not happen in the future, the second is a modal verb "in the past", something didn't happen in the past (and it never will).

Will have + past participle = Future Perfect
The Future Perfect expresses the idea that something will occur before another action in the future. It can also show that something will happen before a specific time in the future.

Would/might/could have + past participle = Past Unreal Conditional
The Past Unreal Conditional is used to talk about imaginary situations in the past. You can describe what you would have done differently or how something could have happened differently if circumstances had been different