There is evidence for St Paul himself having long hair, if the vow Acts 18:18 speaks of is the Nazirite vow.
Right. At least we know, from Acts 21, that his vow ended by his shaving his head in the temple.
Well, no. We know from that very passage that he shaved his head right there in Acts 18:18, but that's beside the point. These vows were apparently common, giving us evidence that there were more than a few first century Jews walking around with long hair.
St John the Baptist would probably have also never known a razor, either, given the obvious parallels of his life with that of Samson (though long hair is not explicitly mentioned in the passage, the wine statement implies that he would have been under life-long Nazirite restrictions).
I don't really see the parallels. St. John ate locusts and honey and wore haircloth and lived only in the desert, none of which is similar to Samson. As you said, there is no mention of his hair.
Hmmm. Miraculous birth, announced by an angel, prescription implying an obvious life-long Nazirite vow. You're right. There are no parallels to Samson. My mistake.
A blanket condemnation of long hair on men seems out of place given that this was still a common practice at the time St Paul would be writing his epistles - it seems that if St Paul was speaking of long hair, he would have at least made some provisions for vows and so on.
Certainly it wasn't St. Paul's purpose to outline in detail hairdressing for Christian men. The passage is brief and provides no room for that. In fact, there's nothing prescriptive in the passage at all about hair -- St. Paul is using hair to make arguments for covering and uncovering of men and women. He does not say, "Women shall not shave"; he makes an argument from the assumption that women would not want to shave their hair. He does not say, "Men should have hair of such-and-such length or styling"; he makes an argument from the assumption that manly nature is incongruous with womanly nature in re the hair.
The English translation under discussion - "Doth not even nature itself teach you, that, if a man have long hair, it is a shame unto him?" and all who followed the Authorized versions translations. It seems that if this was St Paul's meaning, it would have been easy for some sarcastic Corinthian to say, "Gee, what about yourself, Mr 'Apostle'. Why should we honor your words when your very head dishonors you, St Hairy McHarrington?"
The idea of adornment instead of length fits the scenario much better- and let's face it, the English translators of the Bible from Wycliffe on were never much for comparative translation, often preferring to translate things in a light that would cast shade on accepted Catholic practices with little regard for how other non-biblical authors might have used the terms.