Unfortunately, the documents identifying Corbyn as a (probably unwitting) source for Czech intelligence do appear to be genuine. I think this casts irredeemable doubts about his character and lack of judgement, not to mention his wisdom.
There is of course another problem - which is that right-wing press have cried wolf so many times over Labour leaders being dangerous commies (recall, for example, the cynical smears published about Ed Miliband's late father, and also much of the coverage about Gordon Brown), that when a genuine one is actually at the helm the accusation simply doesn't seem to stick in the way that it should.
It's a very bleak situation.