I noticed that the dragons in the Hidden World quickly acknowledged Toothless as their king after his little dance with the Light Fury and a roar. On what basis did the dragons acknowledge him if they weren't present for his fight against Draco's Bewilderbeast
You say "the dragons in the hidden world quickly acknowledged Toothless as their king after his little dance with the Light Fury and a roar."
However, Toothless had already been in the Hidden World for at least a day at that point - he leaves with the Light Fury, they go to the Hidden World, then we see Hiccup at night, and then during daylight Hiccup and Astrid set off in search of him. We do not know if this is the next day, or 2 days later, or even a week.
This throws an alternative explanation on the scene - this is not Toothless doing a little dance and a roar, and being acknowledged as King of Dragons. This is the already-acknowledged King granting his subjects an audience. He's already done whatever he needed to do to be acknowledged as King off-screen.
Well, the Light Fury obviously was a leader in the hidden world and all the dragons who followed the Light Fury recognised that Toothless was the Light Fury’s mate. I think maybe because they sensed that Toothless was male and the Light Fury didn’t object to him running the place.
Also, even though dragons from further away didn’t witness Toothless becoming the Alpha, they all sense that he is the Alpha, so even if the Light Fury didn’t have a high status in the hidden world world, if she was with the Alpha and accepted by the Alpha she would have become important too.