[3] is certainly not 'ungrammatical', and could be a headline for an article in the school magazine, but is clunky as a title.
[2] is arguably ambiguous, as '50th' can of course mean 'the latest of 50 such ...'. However, pragmatics plays an important role here. 'Jill's 10th birthday party' would almost certainly be understood as 'the party to celebrate Jill's 10th birthday', although 'That's his third birthday party this month!' must of course have the other, [third] [birthday party], sense. But I'd not discount the punchy '50th Reunion' on these grounds unless there are hidebound logicians / pedants who might object. People born on the 29th of February doubtless speak of their 50th birthday, even though 50 isn't a multiple of 4.
[1] uses 50 year attributively. And yes, it is far more common to use the singular form, especially with units:
$ a 10-year-old boy
$ a ten-foot barge pole
$ a ten pound hammer
$ a ten cent cigar (note the different hyphenation patterns available/normally used)
Plural-form attributives, as Cerberus details in the linked thread, are used especially to disambiguate (a single bar ??), and I'd come down slightly in favour of [4] 50 years Reunion here, as I feel [1] 50 year Reunion suggests more strongly that there is a pattern of such reunions every 50 years.
I don't think a hyphen would add anything of value, and prefer the decluttered version. So definitely no obsolescent apostrophe here, either.