>What's the practical difference between 'society-wide public ownership' and government ownership? The government is supposed to represent the people.
Typically, socialists (and even market socialists) contend that the current democratic establishment does not represent the people, usually going off Lenin's conception of "bourgeois democracy". This is also behind the most common Marxist rejection of achieving socialism through non-revolutionary means. Compare, for instance, how Lenin described what he figured Soviet Socialism would turn out to be - control by the soviets, not by the state. Of course it didn't really turn out that way, and in Economic Problems of the USSR, Stalin effectively admitted as such.
>What are some existing, real, sustained examples of 'social ownership' in real history?
This is a difficult question because one would either have to draw from pre-capitalist modes of production to answer the question, or one would have to argue that the USSR/Stalinism implemented such social ownership. I have already described why the second is not appropriate (see here[0] too) but the first is also not appropriate because "social ownership" is not a transhoistorical term, it was intended to describe a future mode of production in which it is one of the components of socialism (market or otherwise) - its applicability, therefore, does not encompass pre-capitalist or capitalist modes of production. For the notion of the critique of transhistorical categories, see Marx's discussion on the "Robinsonades"[1] and Patrick Murray's argument on Marx's improvement over David Ricardo and Adam Smith[2]. That said, you would probably have to look deeper into anthropology to find if such a society did indeed exist - that is to say, a society in which as far as we can tell today the will of the producers themselves was actually reflected in the ownership and control of the means of production and its products.
Finally, a note: Marx himself, to my knowledge, never used the term "social ownership". In fact, his argument against Proudhon was that plain "social ownership" results in a state where "society becomes a general capitalist". The term was used by Engels once, but he quotes Marx as follows:
>Marx assumes on page 56 “a community of free individuals, carrying on their work with the means of production in common, in which the labour-power of all the different individuals is consciously applied as the combined labour-power of the community[3]
As such, I tend to avoid using the term, as do most academic Marxists.
[0] "On the Question of Soviet Socialism" (2011) by Paresh Chattopadhyay, a rejoiner to David Laibman's thesis of the USSR having achieved "formal" socialism but not "real" socialism; (Chattopadhyay doubts that it even achieved "formal" socialism in the Marxian sense). https://www.jstor.org/stable/25769087
[1] https://marxism.csbs.utah.narkive.com/qeQK04SD/marx-on-robin...
[2] "Marxs “Truly Social” Labour Theory of Value: Part II, How is Labour that Is Under the Sway of Capital Actually Abstract?" Historical Materialism, Number 7 (Winter 2000)
[3] "Anti-Dühring" by Frederick Engels (1877), chapter XIII. Dialectics. Negation of the Negation; available here: https://www.marxists.org/archive/marx/works/1877/anti-duhrin...