Priestess of Swafnir

Is she wearing some sort of form-fitting coat of plates or leather-covered mail?

A coat of plates would have rivets like these to attach the plates on the inside of a fabric or leather garment, but there would be a lot more of these rivets and they would not be in those places; they would be near the edges of the small overlapping plates. There are only a handful of possible layouts for these plates in the case of armor designed for humanoid anatomy.

A leather garment is clothing, not armor, because it does not protect you. Whereas, a coat of plates would. Historical "leather" armor is in fact plate armor that photographers misinterpreted without looking at what's underneath the leather. Tanned leather was never used for protection, but rawhide (which is much better than leather for this) was occasionally used to make overcoats worn over mail, and in the modern period, buffcoats like Darwin's.

Vests, jacks or jerkins wouldn't have rivets without plating underneath. The rivets alone would offer no protection and add unnecessary weight.

The misconception of "studded leather" armor was first picked up by modern fantasy authors who were looking up image refences without proper context. The concept then gained traction in hollywood because it is easy and inexpensive to make for costuming purposes.

Studded leather is to armor what fishnet fabric is to clothing.
Except that nobody in their right mind would expect fishnet clothing to improve their live expectancy or provide any sort of practical benefit.