OK. Cowboy tucking will prevent kneeing yourself in the face but it doesn't fix the underlying issue which is that you don't actually know how to "flip". From what you describe your somersault take-off is completely wrong and that is why you have no control over your rotation. If you cowboy you won't knee yourself in the face but you could still break your neck because you basically are spinning round with no control, probably very poor height, and no idea what way up you are.
If you want to learn to somersault on a trampoline join a trampolining class where you can learn to do it properly. (It doesn't involve swinging your knees up.)
Try a back handspring first, I've found it helps. Or when you flip backwards, instead of tucking, keep your legs straight and perfect your back layout first. And I've found that bouncing a couple of times before doing the flip and swinging my arms help a lot. When you attempt your back flip (which I admit takes quite some guts to do), make sure you untuck quickly in your case. Hope this helps.
This is a very common problem in gymnastics. I have seen many people break their noses and give themselves black eyes because of this issue. The way we fix it is by using a "cowboy straddle". To imagine this position, first sit on the ground with your legs tucked into your chest, then separate your knees wide enough so that your face will fit between your knees. This way you can remain tucked and still avoid any unpleasant face breakage.
Yes
It takes time and lots of practice before you can do a perfect flip
You don't have to do it.
