We've seen at least two successful ghost-juice uses, one failed, and one non-mortal similar situation.
Mort's Ghost Lease
When Mort uses Harry's ghost, he gains his power and knowledge. But he doesn't 'consume' it, leaving it as a separate entity from himself. Harry detaches from Mort, and there's no reason to believe that Mort retained any power or knowledge from the exchange. He temporarily borrows it, but doesn't join it to himself.
Harry's Nightmare Meal
When the Nightmare ate part of Harry's 'chi' in GP, he consumed it and made it part of himself. He gained the power and knowledge which Harry had. When Harry ate the Nightmare, he regained his own power and knowledge, along with that which Nightmare had. He proves this by using dark power and spells which he didn't know previously. And he never releases that energy to reform as the Nightmare; he consumed it, used it, and apparently burned up whatever energy wasn't originally his until it was gone.
Darkhallow
I imagine the Darkhallow is more like how Harry & the Nightmare consumed each other. The person that consumes the power of the vortex is taking in the power and knowledge of all of those spirits, and joining it to himself. The difference between the two is location. The Darkhallow ritual takes place in the real world by drawing ghosts to the real world and bonding them to a real body. Harry & NM both created/attacked in pocket dimensions in the NN, which allowed them to attack and consume the pure spiritual energy of the other. No vortex needed.
Death by Darkhallow
The way I reason out life in the Dresdenverse is that there are three parts: body, soul, and spirit. The body is physical, and can die. The soul is power/knowledge inherent to the individual, and is eternal. The spirit is natural power tied to the magic field in the Dresdenverse. It can contain some knowledge, but it's not all knowledge like the soul. When someone dies, their soul would go on to whatever domain they associated with. The body would break down, and rejoin nature. The spirit would rejoin the magic field, unless a ghost is formed of it instead. In traumatic death, the spirit doesn't rebond with the magic field because it's unsettled. Instead, it retains partial knowledge from the soul, enough to retain part of it's old living identity. When we see a ghost in the Dresdenverse, it's 100% spirit. Except for Harry, who I think was a spirit/soul combo. At the end, Harry's spirit was all but burned off, which is why Mort couldn't see him, but he still existed as soul. And Mort can't see souls, because he couldn't see Harry or Uriel.
In this interpretation of mine, the Darkhallow would effectively rip the spirit from all of the living things in range. The body would die, and I believe that the soul would move on like it normally would. The spirit, instead of rejoining the magic field, would be merged with the Hallower's spirit, thus increasing their power. Any significant knowledge would also transfer, although I think generic knowledge (like where an old lady keeps her spare key) would be disgarded by the spirit, and only be retained by the soul.
The reason I don't think the souls are consumed by a Darkhallow is simple: the post-life pantheons wouldn't approve at all. If someone can steal another's immortal soul to power themselves, then that would really piss off the other Powers.
Bob's Ride-along
We've also seen a form of spirit merging with Bob's ride-alongs. In the Dresdenverse, I'm assuming animal's have souls, or at least some do. Mouse would seem to have a soul. Maybe Mister does, and maybe he doesn't. Tara West wouldn't seem to have a soul, because there was no soulgaze; but that may be because she either has no soul, or it's so different from Harry's that there's no connection.
So how could creatures without souls, ranging between animals and Fae, use magic? Because they're mostly powered by spirit, which is the connection to the natural magic field thingy. A Fae is probably mostly spirit, with either a fabricated or malleable body, and either little to no soul. For Bob, he doesn't have a form, but the motes that make him are his body; it's just not solid enough to exist on its own.
When Bob goes on a ride-along, his spirit is temporarily bonded with that of the host; Bob can likely see the knowledge in them, and maybe even tap their power like Mort can. But he keeps himself separate, without one consuming the other. It's very similar to what Mort does. The question is whether or not Bob could go on ride-along with Harry and take over, or if Harry could tap into Bob's power and knowledge during ride-along. So it may be the same, and it may be different.
Creating a Mantle
I'm not sure about how to create a Mantle, since it's not fully understood so far. It would make sense that a Darkhallow would create a massive spiritual power-up, but I'm not sure that there'd be enough of a singular identity to create a NEW eternal mantle. I think in the DB Darkhallow, if the Erlking had been in the field, his power and mantle would have been pulled up along with everyone else. In that sense, the Hallower would become the new Erlking, which would be A mantle; but I'm not sure if the new power would be given to the Erlking mantle itself, or if it would still be unique to the Hallower. When the Hallower died, I'm not sure that the power obtained in the Darkhallow would go to the new Erlking.
I think Mantle creation would require more power than a Darkhallow, even on the scale we saw before. Maybe I'm off base, especially if it does in fact consume souls. But I imagine a Darkhallow Ascended entity would be near-immortal, with potential to gather further power to themselves, and eventually create a mantle.
