Crikey, I appreciate your dedication to digging that one out! So I will try and expand a bit.
Whether he became a better keeper or not after the Villa game is a moot point, but over the whole season he was very good. Yes, the Villa game coincided with us suddenly stopping conceding goals, of which he was a big part. Maybe the Villa game was a lesson for him, as it was for the rest of the team? Maybe that was the kick he needed to improve his play to the next level? Regardless, it seems more valuable to me to view the entire body of work with Henderson than just selectively choosing parts of the season. And I'm afraid you'll really struggle to convince me he had anything but a very good season with us last year.
I also think there's more to goalkeeping than just stopping shots, fwiw (which is what Jay is talking about with the 14th-ranked thing, post-Villa game). There are games in the back half of the season where he barely made a save but I think anyone watching would say he still had a good game because of his catching, positioning, distribution etc (and don't overlook that - a couple of mistakes aside, he is good on the ball for a young keeper).
He's not a goalkeeping messiah (but I guess he could become one?), just to be clear here. That's why I'd personally have some questions about blowing a big chunk of our budget on signing him permanently. But as I say, he showed himself to be a very good keeper in the Championship last season, and if we can get him for another year then we absolutely should.