Let's face it if the Kaiser was on the throne would there have been a WWII? If the Tsar was alive do you think Stalin could have acted as he did? Who was responsible for the Rwandan Genocide, Queen Elizabeth II?
To be honest, I don´t think that would have made such a big difference.
Then, was it the Commonwealth of England that put that nation into schism and heresy? Who gave Luther the support in Germany? Didn´t the Holy Roman Emperor dissolve numerous monasteries and passed schismatic laws in the course of Josephinism? Didn´t Louis XVI. take an oath on the French constitution, already condemned by the Pope? Was it the President or King of Sardinia-Piedmont who stole the Patrimony of St. Peter? Did the Weimar Republic or the German Empire fight Catholicism in the
Kulturkampf? And Napoleon, the First and the Third, was a Monarch, too! Juan Carlos, the
Most Catholic King of Spain according to title, pressed Liberalism into the till then pretty sheltered Spanish society.
I could go one forever with this. Still, I favor a
monarchical form of state, for the reason you mentioned above and which can be found in the writings of St. Thomas Aquinas.
But much more important is that the nation, may it be republican or monarchical, follows the right principles of governing, being obedient in all matters to the Catholic faith. Everything else is pretty much a matter of taste.
The President of the United States for example is much more a Monarch than Elizebath II. Truly, the term constitutional monarchy does not really apply to most of the so called monarchies we know, they are merely representative monarchies.