O, the thing with pseudoepigraphic material is that it isn't necessarily non-eye-witness (for instance, there are scholars who regard Mark's Gospel as material dictated by Peter, an eye-witness), but not written by the person named (though, again, Mark might well have been an eye-witness, as well as the scribe - if the idea that he was the young man in the 'background' of the events is true.)
It's possible that one, or maybe two, of the actual eyewitnesses might have had input, but highly unlikely given the timings and the lifespans at the time. That eyewitness testimony would have been highly contaminated by the time the accounts were put into writing, decades after the alleged facts.
That wasn't my point, though, my point was that the arguments against the gospel accounts didn't rest on them not being by the people claimed, but their contents being implausible tales of magic and derring-do.
Remember also, that whilst Jesus had 12 named disciples, he clearly had other followers (there is reference to 70 of them on more than one occasion) and some of them might well have authored the documents we refer to as the Synoptic, or even the 4, Gospels.
Again, the statistical chances of more than one or two of them surviving for the forty years required to be present at the writing of the earliest New Testament works, having been adults at the time of the events, is extremely slim. That's disregarding what the intervening time would have done to their memory of the events.
AS for your use of the terms 'grotesque' and 'impossible', the latter is only the case if Jesus wasn't God - something that no-one has ever managed to prove to be the case. As for the latter, what bits of the Gospels fit that definition?
It's not our obligation to disprove god. You need to give more than a book of stories to justify those claims, or we would be out looking for the skeletal remains of the Cyclops and the corpses of his giant sheep because they were mentioned in the Odyssey. IF god was real, those magical events might become plausible - and the improbable lifespans of the apostles, too - but that circular reasoning isn't going to hold; you can't use god to validate the claims and extraordinary events you are using to validate the claim of God.
Which bits of the Gospels fit the definition of grotesque and impossible: casting demons into pigs and running them off a cliff, turning water into wine, curing leprosy with a touch... the first three that came to mind, I'm sure it wouldn't take long to find others. In the main, you can google 'New Testament miracles' for a comprehensive list of impossible things in there.
O.