I've only ever broken a front rib on a mt. bike, but also a kneecap on my road bike.

Falls are falls and IMO no worse on a mt. bike, just more small falls and more often. That goes with the territory.

The more you mt. bike, the better you get at it if only because you get used to riding on a sketchy surface. It can help to ride with other mt. bikers who will do your pace and show you some skills. Skills are VERY important on a mt. bike.

That said, my first question is why are you riding terrain that is as difficult as you describe ?, especially when you admit to not having skills and/or much time on the single track ?. Why not choose some easier stuff that would be more fun ?. I can't believe it doesn't exist.

When I started biking 28 years ago, it was 50/50 mt. biking to road biking and I don't consider myself either, just a cyclist. Now that I'm 61 I do not attempt the stuff on mt. bike I could do when I was 35. I don't like getting hurt, it interferes with the rest of the day and my skill level as well as confidence level is not as good. That just comes with the ageing territory and I'm fine with it.

So keep the C-Dale and re-think where you are riding so the trail isn't kicking your butt.