I hope you are well today and thank you for your questions.
I have two questions, does smush it plugin support lossless compression of png? I see only jpg listed...
This plugin internally uses Smush.It service provided by Yahoo and i think the service supports lossless compression of png. See the following page on more information on it.
Does the bulk feature support compressing all images in the media library without having to manually select? I mean, can just more or less press a button and it will automatically do all images?
Yes you can sumsh all the images from media library on the following path without manually selecting it.
Admin Area -> Media -> Bulk Smush.it