I think we're talking in circles about O'Brien at this stage, but to summarise my view: I'd like to see him get a run next year. Like you, I would have preferred him to Twomey based on what I saw of them both in that U20 team. But I am more sceptical than you that O'Brien's hurling is up to the standard required, and I could understand why a management who have doubtless seen him play far more often than I have, and who have experience coaching him, might decide to try someone else.
As for bringing younger players (e.g., 19 or 20-year-olds) to train with the seniors, it is hardly unprecedented - Eoin Downey played every league match with the seniors last year, Rob Downey played several senior championship games in 2019, and I'm pretty sure S. Twomey got league minutes with the seniors back in 2020. On which hurlers it suits, I would say it depends on a few things: their physical maturity, the short- or medium-term needs of the squad, and whether they will get regular high-quality training otherwise (e.g., a player with an intermediate club might need it more).
On O'Regan, you were impressed enough by his display against Douglas to mention it in more than one post. Other than that, he has a lot of ground to make up. The good performances at U2O which you mention were five years ago, and were largely confined to the Munster campaign that year. He subsequently lost his form and his starting place. If as you suggest this was down to discipline and fitness issues, then I hope he has overcome them both and gets to prove it in the club championship. But in terms of candidates for next year's squad, he is a good way down the list imo.
