(Lila translates that to the Worldbuilding Department's internal terminology as "faux medieval" and "high magic".)

Oh yes, absolutely; Gwen can design the magic system and whatever other points of Villarosa she wants!  In as much detail as she wants!  The Worldbuilding Department will fill in all the gaps and make it work out as well as they can.

Oh, but what if she says two things that contradict each other?  Like, say, horse-keeping is unknown, but there's an annual festival where people ride horses? 

Then there'll be an annual festival where people try to catch and ride wild horses!

... if there's more of a contradiction than that, then the Worldbuilding Department will still try to do their best.  Maybe there'll be ancient magic involved making it work out somehow.  But yes, in extremis, they have occasionally ignored one design specification in favor of another.

All right; she'll try to keep them from having to do that.

As long as she knows where the limits are...

... Her imagination is starting to run wild.  Why not that horse-catching festival?  Or why not a unicorn-catching festival for the girls?  And give the unicorns wings too?  And there'll be trees that bear ready-baked pies several times a year? And - wait, how's she going to get to Villarosa?  Will she just be stepping out of their version of the Gates of Night or will it be something different?

Yes, that winged-unicorn-catching festival sounds fun!

And as for how Gwen gets to Villarosa...

"We can't just send you through a worldgate... or we could, but that'd be a different sort of story."

Which they do sometimes; Lila has enjoyed arranging a few summons matchups.

"We don't want you to be a stranger in Villarosa, after all; we want a story where you're already firmly rooted there and people know who you are."

Gwen nods slowly.  She knows very well how foreigners can easily be the talk of the town.  That was what first got her talking with Pathred, after all.  And she was ready for it at the Royal Music Academy, too, if she'd ever gotten there.

"So how're you going to arrange it?" she asks.  "Are you going to give me a false story about where I'm coming from?  Are you going to say that people just suddenly appear by magic sometimes, like unicorns?"

Lila smiles, intrigued.  "You could certainly build a world like that if you wanted to!  I'd be interested to watch!

"But no, we're going to... There're two stories you could tell about what we're going to do.  You could say that we'll have you be born into Villarosa, just like any other baby, without any memories of your life up till then.  And you'll grow up in Villarosa until you're a teenager, until you're leaving home for boarding school and the plot of the story is about to start - and then, you'll get back all your memories."

Gwen frowns.  She doesn't like the idea of being a child again, especially not one without her hard-earned memories.

"...And the other story?"

"There'll be a baby born in Villarosa who is... not entirely unlike you... but is growing up on track to become the villainess in the original 'canonical' story, that you're being sent to subvert.

"And then, just when the story is about to start, you replace her, in her body."

Gwen's skepticism is pushed out by sheer confusion.

"-- Those're two different stories!  Can't you tell which one is - I mean, whether the baby is going to be me?  You've got to be able to find where my spirit is!  I mean, you brought me here after I died!"

She throws up her hands.

"... does it matter?  If the story comes out in a way no one will know which is true -"

Gwen interrupts.

"Of course it matters!  Are you going to be mind-controlling me into whatever you think makes a proper villain?  Or are you going to be sending me to replace someone else - and then what happens to her?"

Lila nods slowly.  That does make sense, from Gwen's perspective...

"First off, the original villainess is another person you get to design - within limits, but broad limits.  She needs to be recognizably a villainess, but what sort of villainy is up to you."

"... so I can say she's like Great-Aunt Trinn?"

She's sure the nobles in all Maranon are still counting her as a villainess for the Revolt.

"... perhaps.  I did mention it's a genre of story about romantic rivalries?

"But as for soul magic...  I honestly don't know.  I can't cast soul-magic through our windows into worlds like Villarosa, or any other sort of magic either.  I'm sure someone has done it, but I haven't heard what they've found."

"Well, can you ask ?"

Lila doesn't want to ask.  It'd spoil the story.  It'd nail down too firmly a point it's been glossing over.  

But with Gwen asking...

She nods, and reluctantly flicks her fingers to conjure two notes and send them:  one to the Worldbuilding Team and the other to the Observation Team.  One or the other of them should know.

Worldbuilder Juliett Foxtrot, to Psychopomp Lima Ida
Re: soul magic and reincarnation

I am not aware of that either.  As you say, it is irrelevant to the story; also, it is magic foreign to the world.  I can volunteer that spells cast within the world have never detected our Select persons as being different from the original villainesses - but those spells could be being actively misled, as also spells cast in our Select persons' origin worlds have not detected anything special about their deaths.

It also remains possible that we are all thinking inside a framework different from that on which the Will of the Multiverse operates, and therefore asking meaningless questions.

Observer Uniform Fife, to Psychopomp Lima Ida
Re: soul magic and reincarnation
You are not authorized to know that information.

Lila catches both papers in the air a moment after they tumble out of the message-portal, scans over them in a moment, and shows them to Gwen.  She feels almost eager now, with a mystery being teased into her face.  And the Observer's letter implies that someone might know the answer... and perhaps, many epochs in the future, she might learn it too?

"What?  This - observer - knows and isn't telling us?"

Lila shrugs with affected casualness.  "Perhaps.  Or perhaps they're teasing me.  They've done that sometimes.  But... it's an interesting mystery, isn't it?"

Even though she's going to be just as happy mulling it over if it isn't answered for a while.

"Is there - anyone else you could ask?"