1. The RealSense SDK for Linux contains features (e.g Person Tracking) that Librealsense lacks, making it more like the Windows version of the RealSense SDK. The RealSense SDK for Linux installs Librealsense as one of its modules. So the For Linux SDK is like Librealsense but with more features available to you.
2. Have you seen this article about an example setup of the RealSense SDK for Linux and QT Creator? Maybe you could check it against your own project's setup to see if there is something that you have missed doing.
3. Librealsense has some sample scripts for streaming depth. These are stored in the /examples folder of Librealsense.
Full details of the Librealsense samples can be found on the Developer Guide page. Go down to the very bottom of the page and then scroll up to the 'SAMPLES' heading to find the information.
4. The blackness of the sky section in your image makes me think that the brightness of the outdoor light may still be difficult for the camera to handle. So further experiments with the manual settings such as exposure may be needed. Or an alternative depth method such as Point Cloud may provide better results - there is a point cloud sample in the Librealsense examples.