I wonder who decides which is which?

If we're just talking about whether to call it a novella or a novelette,, I'd hope that it's Jim. But my guess is the publisher.  If we're talking about the decision to write a short piece vs. a longer one, I might say the same thing,  but I don't think Jim went through his publisher for The Law, so Jim?.

What about Websters?

Good question, I have always thought it was a matter of the length of the story.  A novella is longer than your average short story, but not long enough to be considered a novel.  The latter might be a distinction without a difference..

I would agree with that description,  but as it's pretty vague and open to interpretation,  it probably just depends on what the author calls it.

Another idea I had was it being set from the perspective of someone on the White Council like Ramirez tasked with keeping an eye on Harry and seeing how he navigates being an "out law." Carlos could keep coming up against examples of Harry using his power for good which contrasts with what he has been told/believed leading him to possibly change his mind or challenge the White Council on Harry's behalf.

Up to now, the only formats Jim has released from others perspectives are short stories and micro fictions. I'm not thinking he'd release a novella from another character's point of view.

Hi Mira,
This new, just opened up last month or so.  There's a link on Jim's page. So far just the six emails. Includes a complete reading list with books, short stories,  graphic novels,  micro fictions and novellas. And a link to a AMA, quite a few questions,  not lot of answers.

I don't know if we're going to have a full-on ST:TOS-Mirror,Mirror repeat, with Mirror!Harry coming to bide in Dresdenverse!Prime, while Harry!Prime goes to Mirror!Dresdenverse...

IIRC, at a signing or interview, years ago, that episode of Star Trek was the inspiration for MM, to the point (now I'm trying to remember what I'd heard...) that either he might have mirror Harry with a goatee like Kirk did or he might have him actually wear a hat instead...
That's why I think it will be a switch, ending the book with a switch back to the proper time stream.  Then a good part of the next book could be Harry still dealing with all the stuff that mirror Harry messed up here.  Since we wouldn't see what was happening here as we only get our Harry's perspective.

At least those are my theories...

Like maybe a Denarian will be involved in this story, but more intimate than Nicodemus showing up with his latest plan to kick of an apocalypse.

Even if I am wrong about Lashiel, if Harry is using his staff to create a sigil of some sort, what would he use it for, as a design for a summoning circle for a specific being or perhaps as a defensive ward?

The only argument I would have regarding a Denarian being involved,  is that they shouldn't be showing up till book 20, unless Jim decides to change the formula, which he can if he wants to.

I'm not sure how accurate the covers are supposed to be, (hat), but what stood out to me was that his staff was glowing orange, not green.  Orange like when he was employing Hellfire as opposed to the green he's been utilizing since his hookup with Alfred and Demonreach.