There was a Kaggle Competition in case you missed it. At least here you can see what kind of features they are using and what models people employed. See https://www.kaggle.com/c/zillow-prize-1
Zillow Offers team was isolated from the Zestimate team. They didn't share algorithms.
AFAIK this is false; Offers began transacting at the Zestimate price in several markets earlier this year
Could be stale information. I talked to them a while back. Still, transacting at the Zestimate price doesn't mean they're privy to the Zestimate algorithms.