It's quite amusing that Corbyn is considered borderline abject and unelectable when all the stuff you listed for May up to becoming PM seems to be ok.She is part of the problem, a major part. Trying so hard to push through an agreement that the EU can legally not allow. Going on and on and on trying to push it when the EU killed it dead before the Brexiteer Reese-Mogg part of the conservative party had it all changed (and killed again).
The EU legally can not budge on Northern Ireland issues either, legally. She can but won't.
She got into bed with the DUP, which has in its little group circle half a dozen terrorist groups. Not just that but their policy makes Trump look sane.
The only reason most people had a sigh of relief when she replaced Cameron was because even though her 6 years as home secretary were ones of incompetence and outright destructive ideas, at least she wasn't Fox (As secretary of defense gutted the armed forces), Webb (Tried to end sick pay with a "IF you're sick fuck you ." attitude), Gove ("Who cares what the experts say." and as Education secretary nearly destroyed English schools) and Leadsom (who pretty much does every fucking interview highlighting how her being a mother makes her better than anyone, also climate change denier until very recently and refused to allow renewables). She was bad and everyone else was worse.
The only reason she's PM now is because Corbyn for many is still unelectable. Many people are saying they prefer Labour to the Conservatives but outright will not want Corbyn in Number 10. They like his policies but things around him (IRA, Hamas, maybe the anti semetism stuff too, plus the "I have no where to sit on train" when he had whole carriages of space to sit on) put people off too. Someone with his policies or close to him but without the baggage is something that would landslide next election.