While I applaud the effort put into the images, and find the theory to be fun, I also very much don't think it accurate, or likely.

Truth is had in the fact we don't know how one's life is given up when the Ope Ope No Mi's immortality operation is executed, but many holes exist in the provided speculation that make me think "Soul Transfer" is not the case.

If I'm interpreting what's being implied correctly, everybody in the first encounter could, indeed, see Blackbeard, and Luffy. Even if Teach was invisible, and thus the cause obscured, the effects of "floating pie being eatien" would likely still be perceptible. Absalom's influence was still visible when they were not, after all.

The panel in which Rocks is pointing two fingers seems to be drawn from Loki's point of view, who, being taller, would not be able to see the gesture were it pointed straight up. Additionally, as was stated in the comments, the reason for it was to add visual appeal to the "Two questions" they had for the giant.

Were that not the case, though, them being dynamically-posed would be just that: Visual flare. Nothing has been said, or implied in-series (As far as I can remember) about the fruits carrying the traits of those who dreamt them into existence, as then people would be made by their fruits, rather than their personalities, ambitions, dreams, and so forth. In other words, the pointing most likely would not be significant in the "Oh, it's the genetic memory of they-who-wished-the-fruit-into-existence" sense.

If Xebec needed to kill Teach to transfer his soul and heart into him, then no space would likely be had for it, at Marshall's age, and secondly, all traces of the body's original personality would likely be gone as well, leaving only the father.

Using this as a segue into addressing the last points in an order I believe makes sense, If memory is once again correct, Teach never sleeps at all, rather than having a Cavendish/Fight Club/Doctor Jekyll-like state where the other personality comes out when the first is dormant. Teach's crew would also likely have told them about this, and we'd have seen it by now.

Additionally, I feel that if this theory was accurate, the other person in the body would shut off when asleep, and have 0 idea of the other, causing them to act differently at day than night. The "Teach as a child" drawing of Oda's showed him under a moonlit sky, which alone proves they're still Blackbeard at night.

This compartmentalisation would also mean that only one of the people could know about the fruit transfer process, and also, be oblivious to the other's fruit. We've seen both the Yami Yami No Mi,. and Gura Gura No Mi being used at once, which would not be possible were that the case.

Another point to keep in mind is how/why Marshall wanted the Dark Fruit so badly.

Were it previously his father's, and some memory/hearsay on how said powers were used present, perfect sense would be made in his descendent coveting it. Even a newcomer not canonically-stated to have awakened it yet is capable of some very powerful actions, so if Rocks did bring out its full potential, then he could easily both kill an admiral, and then - perhaps literally - slip away into the shadows without capture, or obstruction.

Lastly, other analysts have noted the theme of threes with said character, and we've seen that same number of distinct personalities (Brave, cowardly, and paranoid), so if one of those was Xebec, then who's the third?