TV debates are incredibly flawed. It's simply a battle of perception. You cannot reduce serious questions on policy to two minute snippets. Two leaders arguing and shouting over each is not a effective way of scrutiny or a way to present viable choices to the electorate. The process often comes across as unauthentic because it is. Are candidates really speaking their mind? Or rather regurgitating statements well rehearsed in the days before? Even then a good or bad performance in a TV debate should mean absolutely nothing to a well informed voter. I cannot stress this enough
Coverage of the debate itself is amplified by the 'biased' media the next day. Morning papers, morning radio and morning news and TV programmes are key commercial periods which draw the most listeners/viewers/readers. Doesn't that kinda contradict your point about TV debates having minimal opportunity for media bias? If Corbyn has a poor performance or has one slip its't it likely sensationalized by the 'biased' media the next day?
We have an electorate of roughly 46M, the debate itself is unlikely to draw more than 11M viewers average audience (not sure about repeat viewers) and of that 11M watching you'll have people who will vote Con/Lab/Lib/SNP/Gre etc regardless of their leader or any other leader's performance during the debate. Is the audience is really that large? There'll be people watching who won't value a TV debate as a viable source to affect their views. The people who can be potentially swayed through watching the unedited and unbiased version is a relatively small amount.
I'd argue that the Labour campaign and Corbyn is currently seeing a great amount of success with direct interaction with the public and public sector organizations, a considerable amount more success than May or the Tories will ever have in that area. I think these massive public rallies are the best way for Labour to get their message across unbiased, undistorted and to large numbers of people, it's much more authentic and genuine platform to communicate with common people than a TV debate. I've been really impressed with the crowds at these public labour events. Not to mention these events are highly circulated and amplified by social media users, a platform which left leaning voters dominate.