Managers will fight for status quo, its their only thing. If you can not resolve conflict with the manager, quitting is the only right answer.
From the managers manager perspective its too much work to oversee people at 2 levels. If the manager gets the job done, that idle for him/her.
In my experience HR department always sides with the most people who have most power. This is not surprising, they don't have any direct contribution and have much less power.