I think the deal with the turtles is that they have some sort of property relating to the edelwood tree oil or something. That's my guess, anyway.
The Pendleton Ward comparison is a pretty accurate one, since the series creator is someone who worked on Adventure Time and Pendleton Ward was the one who animated the fantastic Cab Calloway moment of the Highwayman's song. (plus as someone who has a complete Beatrix Potter book collection stashed away in my closet, I really enjoyed the ambiguity of if the animal schoolchildren were actually able to understand what was going on or not, since Potter's books tend to have very strange treatments of how much the animals in them are actually "people".)
I think they wrapped things up in a way that it would make doing a sequel series really unuseful, unless they did episodes that are supposed to happen in the earlier timeframe of the show (on the way to Adelaide) or without Wirt and Greg present, but it's also very unlikely that would get greenlit. I would love to see the original pilot animation though, since it doesn't seem to be available anywhere. (maybe they'll make it a special feature on a DVD release or something).
Also, a really interesting thing to do after finishing the series is to rewatch the opening sequence in the first episode in comparison to the closing at the last, because there's some very interesting bookends between them that you won't understand until you've finished the series.