(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.