Good questions! FWIW I'm not convinced there is a trend towards ever-shorter PBs. The main example text we used in the webinar was The Snowy Day, published more than 50 years ago, which at 300 words and change is right in line with the word count that we are told is the "shorter" word count of today!
In fact picture books have a big range. There has been some erosion at the very top end--the picture story books--due to parents pushing their kids out of picture books and into early readers and chapter books as fast as they can (IMO).
It has long been a practice to insulate illustrators from the authors. I can tell you horror stories of what can happen when publishers don't. The practice evolved early in order to let the illustrator work without pressure from the authors. Authors' experience is with words, and though they often have a vision of how they want "their" book to look, it can be a more limited and less creative one than what illustrators can develop. Time and time again, I've heard from published authors how pleased they were to see how a picture book turned out--that it was better than what they had imagined.
Collaborations like the one you mention DO happen, and I think are not a new thing. I remember when I worked at Orchard 25 years or so ago, Peter Catalanotto did a number of picture books in collaboration with George Ella Lyon. But these are very much the exception and I don't expect they will increase.
As for more books by author-illustrators, I know that that's widely accepted as something that's happening now, and it's my impression that the Big 5 or 6 publishers DO tend to go for author-illustrators for their picture books, but I don't think anyone has dug into the numbers. If you compared catalogs from today with 10, 20, 30, etc. years ago, what would you see? I don't know.
(I'm answering here because we aren't going to talk much about the market, except in the final session. We think that it's crucial to make your manuscript be the best it can be...)