Your thoughts on the video?
At around 1:37, he starts talking about the idea that Asta had the reincarnation of the dead elf child instead of Yunowhile Yuno had the reincarnation of the living elf.
That is outright nonsense and would've been nonsense at the time when the video was made as well. We already knew what happened to the living elf; he went on to survive, be born normally, and presumably, along with the mother, stumble upon Elysia where they would have children of their own and found the village of Elysian elves.
The dead one was reincarnated into Yuno. The fact that it was dead would've been why it got roped up in that same reincarnation spell that all of the other elves did. The living one would've lived through the event, thus not getting caught up in it.
At around 2:15, he starts talking about Alan potentially being Asta's father, which is a pretty believable idea, though the segway into Asta potentially getting another grimoire is pretty trash. Especially since he isn't even willing to concede that Asta would so much as gain access to magic as well, should such a thing happen.
For the main theory of the video, the evidence is pretty circumstantial, and certain parts of it would either be too confusing for the readers, or contrary to the main themes of Black Clover, but the core of the idea that Julius has something to do with the fourth triad Sibling, and that their influence has already been present within this arc, is relatively believable.