Perkin Warbeck appears to have been openly accepted as being Richard, Duke of York by the Holy Roman Emperor, Maximilian I as well as the kings of both France and Scotland (and possibly by Ireland). I understand that it might amuse them to tweak Henry VII's nose with a pretender but from what I've read they offered this gent every courtesy, spent a lot of money on him and even married him off to a noble Scottish bride. Henry even had to invade France to force Charles to send Warbeck away. These kings seem to have risked and spent a lot (including their time) if they didn't really believe they were talking to King Richard IV of England.
It is generally accepted, yes, though I cannot say if it is universal; very few things are. But yes, I would hazard that 99% of historians agree that Warbeck was just that - Perkin Warbeck.
As for why the other Kings supported him: King Maximilian simply liked him. Personally, if not politically, he liked the guy, and so enjoyed giving him support. He was reluctant to give up this support after Henry and him signed a treaty in the early 1490s, but ultimately the Pope and the rest of the Holy League made him as they were going to war with France, and wanted Henry on their side. So Max had to give up support for his friend.
Charles VIII of France never really supported him, and never considered doing so. He simply hosted him as a bit of a, like you said, ‘nose tweak’ to Henry. However, he spent no money and gave no support. Henry invaded France in 1492 because thats what English Kings did - they invaded France. His main reason, really, was that he wanted the French pension - £10,000 a year from France. He got it because France was getting ready for war with the HRE and essentially Charles just wanted Henry to ‘bugger off’. The French Pension came with a promise to not support any more pretenders, which he upheld.
Ireland didn’t have a king, and they wanted nothing to do with Warbeck after they had supported and got nothing from the first pretender of Henry’s reign, Lambert Simnel. He found a little support there, but nothing serious.
Scotland was his greatest supporter, but not because he was royal. James IV of Scotland wanted war with England, and by supporting Warbeck he could impose the Pretender as King of England and have a puppet king rule there. He married him to Catherine Gordon to further his support - if he really believed that Warbeck was royalty, Catherine Gordon would have been an insult as she only had a tiny bit of royal blood in her, if any.
Margaret of Burgundy - the aunt of the Princes in the Tower - said of Warbeck that he really was her nephew, but that was more of a ‘fuck you’ to the man who killed Richard III rather than actually believing him to be her nephew. Elizabeth of York, Henry’s wife, would have seen him too, and made no indication that she knew him.
Hope that helps! Love seeing Henry VII questions here. As you can guess from my username.