So why have the Working Class deserted Labour in massive numbers since Corbyn took over?
The same reason that they would have abandoned any of the candidates who were in charge of the Labour Party and the same answer to your points about:
I don't even know what Corbyn's leadership is meant to be achieving anymore, all of the predictions which his supporters made back in 2015 look even more ludicrous now than they did then.
We were told that we needed Corbyn and a move to the left to win back Scotland, results in Scotland have gotten much worse rather than better.
We were told the centre of political gravity would be pulled leftwards, it hasn't, its now pandering to batshit crazy nationalists, seemingly with the tacit consent of Corbyn when it comes his old bugbear, the EU.
The reason is that politicians, Corbyn included, have not yet adjusted to the new reality of Left Vs Right being outdated as a prism by which to look through things. The larger gap is now between neoliberal globalisation vs protectionist nationalism and these fault lines traverse social classes.
Unfortunately, the level of political analysis and discourse is so dismal that people tend to equate whatever they happen to like with how “Left” or “Right” wing it is. Corbnyn? “too Left”. Cameron? “too Right”. May? Oh God we hate here – she’s even MORE Right wing.
In reality the “May doctrine” (if she has been in the job long enough to deserve the moniker) is her electing to move “leftwards” economically and “rightwards” socially. Thereby she is navigating the issue of nationalism as all of the most successful politicians are doing (Sturgeon, Trump, numerous European examples).
Any successful Labour leader needs to come up with a counter narrative that can do the same.
It's funny how Corbyn's views on greater party democracy and responding to the wishes of the membership go out the window when its the Brexit issue where his views happen to fall in line with the Tories rather than his own members.
The leadership has to be decided by the membership. Once he (or she) becomes leader they have to be accountable to the populace if they have any hope of gaining power. The nation has clearly voted for Brexit.
And anyone who thinks that leaving the EU under this government is likely to lead to greater socialism in the UK should probably seek professional help.
Whether it will or it won’t is a separate issue to whether it can.