I second the idea that you check out the turtledog portable stand thread imagine whether you could make one to use in the yard, in the basement, in the den, the garage, at grandma's. This may require you to temporarily postpone your own preconceived notions of what you really want for your bedroom. But the compromise may prove very enlightening about where else you could hang for now. You will instantly become the envy of your friends with a hammock and a Turtledog stand. Mom should appreciate your willingness to compromise, and then may soften amazingly fast when she sees the mellow bliss that radiates from the hammock from all who test it.
Show Mom a few of the more attractive photos, reassure her it can disappear in three minutes into a closet, and wouldn't require you to throw away the bed or scar the room in any way.
You found your way into this forum to ask for help. I suspect you will persist to success with your Mom before too long.
Good Luck! Keep us updated.