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Check out the rosbag Code APIs. This will allow you to isolate messages from specific topics from the rosbag.

You can use either C++ or Python to process your rosbags and point clouds. I'm not familiar with getting velocity or intensity data from PointCloud2 messages but in general I find that if you want more than x,y,z data from the point clouds then it's better to use C++. The Python API for accessing point clouds data is more limited. Check out the Point Cloud Library (PCL) for processing point clouds.