Running navigation stack with odom, IMU and gps

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As mentioned by @soldierofhell this is more a "localization" problem than a navigation problem. In order to combine odom, IMU, and GPS to generate an estimate of your position consider using the robot_localization package. This, as far as my experience is concerned, is the standard way to do what you want to do. Once you have your robot localized in the space then the ROS nav stack will be able to take you from waypoint to waypoint.

For the integration of GPS robot_localization provide the navsat_transform_node and a detailed tutorial on how to integrate this with the other sensors to generate a localization state estimate.

1. It's more of a localization problem, then navigation. The precision of localization depends on quality of your sensors
2. If you're not afraid of obstacles you don't have to use them in planning or you can route a predefined paths (assumed your localization quality is fine)