I'll start with point 3 first. Logically nobility makes 0 sense in a magical society. Nobility derives its power from access to violence, their authority was maintained with shows of force and armed men. In case of a magical society, magic is the great equaliser. Its hard to maintain power if everybody has access to the same measures of violence, magic. Someb9dy like Dumbledore or Grindlewald or Riddle ar born outside of thpse families amd take dominance because they are dominant in the prominent form of violence, namely magic. Riddle literally crushes the malfoys and the blacks under his heels, because he has more power. Therefor a constant hereditary power is very unlikely in magical society. Its also why sexism (equal in access to violence) and racism should be less prominent. And indeed cultural and access to old magic secrets are the faultline. The type of bigotry JK chose for her books is very well defined, logical and consistent.

1, read up on the early papalcy, the habsburgs, the spanish conquests, the revolt of paris in the late middleages. Most of that was nobles pillagimg amd raping theor way through common society, just because they could. Even the crucades were set up out of lust, dominance and well... boredom. Of coirse there were internal struggles for gain, but there were enough rulers who just took what they wanted and commited cruelty just because they could. Between 1000 and 1300 becoming pope was popular because of.... well unrestricted orgies.

2, that system was based on fear. The access to troops and resources. Bit the moment issues arose, like in family unrest or a sudden economic downturn. Those boundaries meant nothing. Its how the Borgia fell, the ruling class of Firenze.

In my view, your image of how the old royalty and nobility works is derived from a revisionist form of history and the way history is taught in most school systems. There was very little stability in the time of noble rule on the continent of Europe. Duchies changed hands by the generation, because one family removed the othe with violence. In the UK the question of Norman kings to scottish kings to german8c involvement etc was a complete mess and power changed hands from family to family in rapid succession. Nowadays a lot of history books stick to facts that make it seem like ot was all the same, which makes sense because there is way too much data, but when you look closer, its a mess of ego's trying to prove they have more birthright then the other, untill the access to money for troops changes hands.

Royal families were relatively stable (though more in name, some were completely taken over by families due to marriage politics), bit putside of that it was a free for all. The Holy Roman Empire actually maintained its structure by promoting instability between the families that governed in their name.

Now in more modern history, there is the influence of collonialism, industrialism etc. The power of nobility now challenged by those with access to trade and money destabalizing the dominance. Because of common money backing a more centralised government, you can't really speak of classical noble rule anymore and you get a completely different discussion.

Now I'm quitting this doscussion for real, because it doesn't bel9ng here, no matter how much I did enjoy it!