1) We have investigated this issue a lot and are not able to replicate your problem even when we switch back to Groovy. Can you please post a gist link of your full build error of gazebo_ros_pkgs by running in your workspace catkin_make VERBOSE=1?
3) It looks like you are mixing your installation paths up... that error output you posted references both /usr/local/include and /usr/include/ - make sure you fully uninstall Gazebo then try installing it again to just one location.