I'm not sure why you think we're using bootstrap, we're not. Everything in our code base is bespoke (with the exception of jQuery 2). As a result, we get to choose our breakpoints, and they're flexible for now (there's a lot of movement before we go to the next one).
The iPad in portrait mode doesn't have the available width to show the second column without cramming the stories in the main column.
We settled on the breakpoints we have due to the amount of devices we want to offer our content on. We're a service that's trying to work on as many devices as possible, from Nokia 100s to modern desktop browsers. So we did the logical thing of splitting devices into groups, feature phones, smart phones, tablets, and above (desktop & TVs). Some of the larger screened Android phones cause issues since they have a massive resolution, but can't display content as a tablet because it would be hard to read.
You're right, we don't have volume settings right now, so we can't emphasise any of the stories except to make them the "top" story. We haven't got to that point yet, we are after all in beta. All I can say is it will come.
I can't comment on editorial context, just the technology.
The most read functionality is being worked on to be richer.
Thanks :)