You're showing an arbitrary HTML snippet here, not Bootstrap's grid system. Add the grid markup and make sure your image is responsive, and you'll get https://jsfiddle.net/q39rq6eu/6/.
Not true, see here: https://jsfiddle.net/2om8t67a/3/ Images act a bit weird in flex though, so it might be true for img elements as direct children of flex containers, but for normal content it's not.
This is not entirely accurate and I know it as the developer of Picnic CSS ( http://picnicss.com/ , which has been using flexbox for a while).
The columns will only be equal width with the same content or some smaller content, similarly to how table works. Case in point:
https://jsfiddle.net/q39rq6eu/
If the columns were equal-width, we'd see in the example that the one on the right starts in the middle of the screen.