1 why did redyue disappear when ghost taka was around?
She doesn’t. During the first time Mitsuzane hallucinates that his brother is in the room, she stands right before him. During the time Mitsuzane hallucinates while fighting Kouta & Kaito, she watches. I admit, in the current episode it kinda looks like Redue disappears but its either a production error or Mitsuzane cant see her because of his brother. And because we are seeing what Mitsuzane sees, she's not there.
I really like this theory of yours, but it gets totally destroyed if you watch the first time Takatoras Ghost appears. Redue stands in the same room and wonders about Mitsuzane, saying "Are you talking to yourself now?" and she thinks "Is he broken?". Why should she, if she's the one causing the hallucination? And we see a shot where Mitsuzane's throwing stuff at an invisible Takatora, Redue stands there and watches, but she cant see what Mitsuzane is so angry about.
The problem with this theory is, after the fact I just explained, Redue is sometimes there when the ghost appears, sometimes not. If she would be always there or not at all, that would be suspicious. But that’s not the case. Also: If the producers pulled that, why not build it up properly? Why not show Redue as a puppeteer, holding the strings of the ghost? No one knows that she has something to do with it until she's dead? Nah. There are no clues to this theory, and if there would be they would have Mitsuzane kill her to free him in the end. Not just a dead Redue and then Takatora is gone.
I think the ghost is still there, because Mitsuzane is crazy. This ghost is a complex he developed. Maybe he "destroys" the ghost, or rather this form of his subconscious, by smashing the Genesis Driver and Melon Energy lockseed once he has Yomutsu Heguri.
and 2, why did he make that weird helheim noise when he appeared most recently?
Just a simple sound effect. If it would be a thing, they would have used it every time Takatora appears, not just this time.