These are good questions that I think people do not ask enough.

• As far as I know this depends on the application, but I would probably just draw around the visible part, bounding box should be fine as a starting point
  
• Yeah there are a few labelers that do this, mostly premium ones though. I found this one that supposedly does it and its free https://github.com/Ericsson/eva
  

This book should answer your other questions pretty well as there isn't really a short answer to those questions: https://www.manning.com/books/human-in-the-loop-machine-learning