Renditions currently are not supported. In other words, you cannot create an article and upload multiple article files, such as one for phones and one for tablets. Instead, you can choose an option to create different top-level content for phones and tablets. That way, if you use alternate layouts in InDesign to create multiple sizes of the same layout (such as 640x1136 for phones and 768x1024 for tablets), your "pixel-perfect" InDesign-based articles will have minimal letterboxing. The drawback to this approach is that you have to maintain separate collections and articles for phones and tablets. Various rendition options are on the roadmap.
Yes, at this time, DPS 2015 is single-orientation, although you can specify different orientations for phones and tablets in the same app. In the near future, there will be support for including both horizontal and vertical articles in the same app so that users would rotate the device when they need to switch from a horizontal to a vertical article. However, there are no plans to implement dual-orientation articles in which you rotate the device to view a different layout of the same article.