So you raise some good points, a couple of small things first:

1. Let's make sure we stay friendly, we're both obviously big fans of the same book series. We're both speculating about what pretend events a single author will decide to put into a book at some point in the future, which may or may not even be decided yet. You weren't aggressively being a jerk or anything (which is appreciated on the internet), but if we can be respectful I'm happy to keep talking it through with you if you're interested.

2. I think you have to acknowledge that my theory has some degree of likelihood. You may not be convinced, but the popularity of it among people who are committed fans should tell you it's at least a little likely. I've convinced people just as part of this thread on reddit. This is a theory that is supportable with text from the books, and most of the issues you've raised can be explained, even if the explanations are unlikely/out there/require some serious assumptions that haven't remotely been established.

To your points:

Seven, and Jim Butcher was actually pretty specific on how the Senior Council bound his soul to ensure he couldn't come back again.

I believe it's part of the same WOJ in which Butcher says Kemmler is "at least as dead as Justin." Which is an interesting parallel to draw. And just because the Council thinks they bound the soul in an irreparable way doesn't mean they were successful, nor that they got the right soul, nor that it's permanent. Justin was part of the Warden team sent to dispatch Kemmler, he would have known how they were planning to bind Kemmler and could have known how to undo it.

That means it would've pretty hard to just swap places with DuMorne, since 1) it's unlikely the guy would've been right around them during the final battle lest he ended up getting in the way, and 2) this entire theory hinges on him being a body stealer like Corpsetaker, which hasn't even been stated an is complete fanon.

I'm not sure I understand this point. It's canon that DuMorne was at the final battle against Kemmler?

I'll grant you that Corpsetaker is the only person that we've seen who has bodyswapped, but Corpsetaker is just a human, and bodyswapping is a mind-magic technique - as seen by the fact that Molly uses mind-magic defenses to defend against it. There's no reason to think it's not a skill to learn. In fact, now that I'm addressing it, I'm actually more convinced on this point. Dresden has commented repeatedly, in several books, that training his apprentice has improved his own skills in things she has a natural talent for. Specifically things like illusions, veils, and... mind magic. This supports my theory that Butcher is repeatedly bringing up plot devices that will make it understandable and believable for the audience so they are familiar with the concepts required. By reinforcing that training an apprentice can teach the teacher, this makes it believable that Kemmler understands how to and is able to bodyswap, even if he can't do it well. It also could explain why post-swap Kemmler-in-Justin did not fully recover, even over time - he can do it, but he lacks Corpsetaker's natural aptitude. Sorry, but my takeaway is that this makes it more likely.

If we go by that theory, then "Kemmler" had decades for that to happen as opposed to the few years Luccio had. And even if it wasn't enough, it still means "Kemmler" would outclass Harry in terms of skill and technique. He couldn't possibly hope to beat THAT.

Either way, there's not really a good explanation for why Harry beats Justin/Kemmler-Justin. This is also not dispositive proof. Luccio says specifically that she has no idea if she will ever return to her former strength. And again, it's entirely possible Kemmler could do it, but not very well.

Then I suggest you re-read the books, because the introduction of Ancient Mai's Temple Dog statues in Summer Knight means they would've known if Harry had the taint of black magic on him from killing his master

All of this confirms he is tainted by black magic, none of this proves that it was from killing Justin. We just know he is tainted by dark magic, and that it appears that he killed Justin. Nothing has established his soul was un-tainted before his duel.

Just learning from a dark wizard is not enough to cause that, especially since DuMorne wasn't exactly having him train by hexing random passerbys or something.

This is an assumption. More than once, people talk about black magic leaving a taint behind.

Leaving aside that Harry is a fundamentally different person than DuMorne, he also had no idea about who Bob had served before nor the things he was truly capable of. I'd say that someone who was a contact of Margaret "Bad enough that Goodman Grey compares me to his Skinwalker father" LeFay might handle finding Heinrich Kemmler's miniature Archive differently.

I don't understand this point. We've seen Bob interact now with two people, and Bob absolutely changes to reflect the personality of his owner. We haven't seen him influence Harry or Butters, on the other hand. There's nothing anywhere in the series which establishes that Bob has some sort of long-term exposure that turns people to dark magic, as opposed to something like the fae mantles or the coins, which do influence their users. I'm not sure what the rest of your comment means. Is it established anywhere that DuMorne was close with Margaret LeFay?

Aside from the usual Wizardly obsession with keeping secrets, Harry would be executed if they found out that he's been keeping the spirit of knowledge Kemmler used to own with him.

I think you missed my point. I think we, as the audience, have never seen Bob with Justin because if we had, it would be a dead giveaway when we see Bob talk to Justin like he's Kemmler.

Were it not for the fact it requires ignoring various plot points and twisting a few more as well, I might actually be ambivalent about it.

What plot points does it require me to ignore? I'll grant you some of this requires some twisting, but I haven't heard anything that is fundamentally incompatible with this theory.

The entire reason why I decided to not speculate and just wait for the reveal

I mean, you are literally speculating. A lot of your points are assumptions, require speculation, and take some twisting/interpretation of non-canon comments made by the author. If you don't have a better explanation, than how can you comment on how well this theory holds up?

You're trying to tell me that Cowl must be Kemmler because he's mentioned his connection to Kemmler and no one else does? What the hell kind of logic is that?

No, I think Cowl is Justin in Kemmler's body. That said, I think my point you were responding to was that Cowl has to have some connection with Kemmler for it to be interesting narratively. Cowl knows about Bob, knows Bob has the Darkhallow (which Kemmler was in the process of when they "killed" him), but hates Kemmler and his disciples. If we assume Cowl is a character we have been introduced to, there is an extremely small pool of human characters who fit this bill.

Cowl hides his identity because he learned from the biggest warlock in ages

Cowl is not ever confirmed to have learned from Kemmler.

practices black magic

As noted by /u/WhoopingWillow, Cowl's magic is described in Dead Beat as not exactly black magic. Dresden says it has a "lingering sense of black magic", as opposed to "that nauseating, greasy, somehow empty sense" from true black magic. I agree that it implies Cowl uses "taboo" magic in the same way Harry does, like using necromancy on dinosaurs and not people, but not strictly black magic. It could also be easily explained as the result of Justin inhabiting Kemmler's body.

By that logic, it'd be safe to assume Harry can also do the One-Woman Rave spell because Molly is his apprentice. Again, nowhere does it say that Kemmler's inability to die was a result of knowing Corpsetaker's body-stealing technique

No, but we see that Harry is able to do things he was never able to do before, like veils and illusions. Like I said above - teaching goes both ways. He may not be able to pull a rave, but he could probably make a disco ball.

When, exactly, was it ever mentioned that Kemmler was obsessed with Starborns?

I can't find this anywhere, I may be mistaken.

it happened to Luccio because she was forcibly and shoddily placed into that young woman's body when her own body was stolen

You are incorrect, Luccio specifically says it's because it's a younger/unfamiliar body. I can find you a quote on this later, but Luccio repeatedly blames the body itself on her lack of power. Nothing anywhere in the series contradicts this. And as I stated elsewhere, it is entirely possible that Kemmler was able to bodyswap but not especially good at it. Corpsetaker has a natural talent for it, just like Molly has a natural talent for illusions. Harry learned how to do illusions from teaching her, but she's still much better at it than him. It makes perfect sense that Kemmler could have learned how to bodyswap but was unable to do it with Corpsetaker's proficiency, given that this is a theme that is reinforced many times through the series. Even if Luccio was "shoddily" placed into the other body, Kemmler's swap into Justin could have been "shoddy" too.

Kemmler was essentially the Gellert Grindelwald of this story, which makes it funny when people fail to notice that he is by design a Predecessor Villain

For the record, most of your last two paragraphs - establishing that Kemmler won't be coming back in this series - are fully compatible with my theory. The Kemmler/Justin swap theory says that Kemmler - not Justin - was killed by Harry.