But all these features were also true for the dictatorships toppled say in the Arab Spring. Or Franco. People were not expected to be engaging in politics, support was not manufactured etc. Still there was unrest and instability.

Putting it differently, from the Aristotelean stability-first angle the question is why and how would people accept it, when there is empirical fact they don't accept it in dictatorships.

As far as I can tell these kinds of demonstrations and unrest have two factors. One, students, intellectuals who care about things like freedom of speech: basically, with some cynicism you could see it they want a piece from the power cake. Perhaps a system that would offer them clear paths to power could defuse it, but being rebellious still feels more virtuous and empowering than repeating official propaganda for a chance of promotion and a sinecure so the only system I can imagine that could secure their support would be itself pretending to be perpetual rebels: welcome to the "Cathedral". Lacking that, you could shower honor and money on young intellectuals and still they would find rebellion more virtuous and empowering. A second factor is the basic simple hunger-revolt urges of the masses when and if the rulers manage to screw up the economy. You could see both factors in the Arab Spring, the mass-hunger-revolt being the muscle doing the pedaling behind it and the rebellious students and young intellectuals the steering brain.

It would be fascinating to do an in-depth study of student and young-intellectual rebelliousness. It looks like something invented in the 1960's, but Stefan Zweig in The World of Yesterday mentioned it existed in Vienna as far back as his youth1900, but weirdly enough, it was a proto-Nazi type of student movement, basically nationalist students getting drunk and starting fights in the name of some pan-German union. One of the weirdest and most scary facts of early 20th century Europe is that students were above-average likely to participate in proto-fascist movements. From these two data points one could speculate that it may be an ancestral urge, basically young males not wanting to be ruled by the silverbacks, and ape or caveman level status competition. Around the world, youth radicalism was visible in 1908, visible in 1848 and so on. Any monarchy should need a surefire way of dealing with that (i.e. give them power and prestige but also make it as romantic, virtuous and empowering as a revolution) to be seriously considerable.

Buying votes is illegal.

Come on. Spending money on making a candidate or party attractive and advertised buys votes. Not literally but in the sense of increases the chance of people voting for them.