Agree with all of that. Corbyn performs well in his comfort zone - campaigning, canvassing and also being on television in either a) a panel format or b) the Sunday show on the sofa format. So if he maximises those opportunities he'll surprise a few people for sure. May is incredibly poor at debates so Corbyn will probably beat her - he just about manages (lol) to do it most weeks at PMQs, which is testament to how AWFUL May is rather than how good Corbyn is.
Problem is - in 2015 the more voters saw of Miliband the more they liked him. And they rewarded him with a Tory majority. Obviously that had a lot to do with the Labour/SNP coalition worry in rural England. But I think the stuff the Tories have in their locker for Corbs is worse than that, tbf.