In my pseudo-medieval world, in the culture/religion of one of the largest and most powerful nations, woman are thought to be generally better suited to positions of power (as men are believed to be usually too impulsive, emotional and narrow-minded to wield power effectively). This is a very soft precept though and men definitely can and do achieve high success/social and political standings in virtually all fields (especially the military).
The exception is the most important position of power: the ruler of the kingdom must always be a woman. This role is hereditary, it passes down from mother to daughter (or in the event that there is no daughter, to the closest female blood relative).
Though this dynastic sovereignty is sanctioned both in the minds of the people and in this kingdom's theology, it is also deeply held that the ruler should never be allowed to become prideful and arrogant.
As such for centuries measures have been taken to avoid the latter:
She has no official title associated with her being the ruler. She is simply called "Lady" much as any noble woman without a rank would be called.
She is expected to wear simple clothes, no gold, or mink fur, etc, again of a similar style to what any daughter of an aristocrat family would wear.
No noble people will ever bow down to her and peasants show the same respect as they would to any low ranking noble-born (not that she spends much time with peasants, mind you), perhaps a simple low nod of acknowledgement.
She has no special throne or crown.
There is no special ceremony when she becomes ruler.
She is expected to never directly enact her authority, at least not publicly (no ordering people to do things).
She cannot talk about matters of the state in public or discuss such matters with foreign dignitaries (at least not directly).
Now obviously the two last rules need some sort of work around, otherwise she would have trouble commanding her nation:
That's where the role of king becomes important.
The king acts as essentially a substitute/avatar/mouthpiece for the ruler:
Instead of the ruler ordering someone's head be chopped off, she would whisper to the king to do so.
Instead of a foreign ambassador talking to her, they'd talk to the King and she'd listen. Whenever an important response needs to be offered, she and the King would go to a private chamber so that she can tell him what to say.
When a neighboring country wants to show their loyalty or amicability, they'll send an expensive and elaborate gift to the king (and specifically for the king too, I don't mean he'll act as a royal postman accepting gifts destined for her).
When the various consuls, proconsuls, ministers and generals, need to be told what to do, it is the king who delivers the instructions.
Royal speeches are written by the ruler (or one of her advisors) and then memorized and delivered by the king.
Senate meetings (yes there is a senate, it doesn't have much power though), are attended and overseen by the King (because constant whispering is obnoxious in that setting and the King and ruler can't leave the room every couple minutes, typically she'll discreetly write what he should say on a piece of parchment in front of him).
The only exception to all of this is meetings between the Ruler and her advisors, in these she will talk freely (the king sometimes comes along too, but there is no reason for him to speak).
In addition all of the high honors that the ruler is barred from are not in the case of the King:
He wears extremely expensive clothing.
He has a throne and crown.
Everybody definitely does bow to the King.
Everybody uses a myriad of flowery epithets when addressing the King (majesty, greatness, etc..)
When he becomes king (when he marries the ruler), there is a coronation.
Now I'd like to reiterate that this isn't one of these "Behind every great man is a woman" moments. Indeed, even officially the king has no power, everybody is clearly aware that every time he says something he's just parroting the true monarch. Nobody tries to hide the fact that when the King excuses himself out of the room to "deliberate on the matter at hand" it's just code for "I need to be told what to say".
Even neighbouring countries understand perfectly well that he's basically just the ruler's partner in fancy dress.
Even so, it's extremely important culturally, socially and religiously. So much so that at some point in this kingdom's history a short, minor war broke out between it and a foreign land after one of the latter's ambassadors adamantly refused to play along.
It's also very important in that the various high families of the realm will vie with each other in order to try and get one of their sons appointed king (for the obvious political reasons but also for the additional social standing it affords to have a family member be King).
So after all that explaining, my question this: is this Kingdom unstable as a result of this unusual arrangement, is it more prone to coups? Social/political unrest?