Your race bike geometry will definitely affect how you feel going downhill. I had a racy type hardtail and always felt like I was about to pitch over the bars on steep descending no matter how far I got back (which turns out wasn't actually that far). When I got my Titus (Moto-Lite not Racer X) the slack head angle was very noticeable, I feel much more stable on steep descents now without having to go all the way back. I now have a non-racy hardtail now as well. I rode my racy hardtail for a few years, it just took more of a conscious effort at weight moving.

How far do you really shove your backside back? I used to think I was way back until I saw pictures of myself descending and I wasn't really as far back as I could go at all. Putting your saddle down, really down, will help this. Maybe someone could take a picture of you going down something steep so you can see your position on the bike and also where your eyes are looking i.e. is it down the trail (good) or at your front wheel (usually bad). Flow comes when you look well ahead and know whats coming, if you don't you get reactive and everythings a surprise (good preparation for night riding though).

As for Switchbacks, well... They're damn hard! I went on an MTB holiday to Spain in September with a company called "Switchbacks". There were loads of them, steep, massive rocks on them and precipitous drops off the edge if you got it wrong. We don't get that kind of thing in the UK. The way to ride them was to hang over the back wheel, keep a bit of momentum and look through the turn where you wanted to go, if you looked at the edge or any of the rocks/obstacles that's where your front wheel went. They also said to change your leading foot depending on which way the turn went - left in front for right turns and vice versa, I naturally lead with my right foot and found left turns ok, right turns were more difficult as I couldn't get my balance with the "wrong" foot in front. I'm going back next year to have another go!

I really recommend trying descending by putting your saddle way down, it'll feel weird at first cause you can't grab it between your thighs really but the difference in control of the bike is immense! You will find it gives your quads a good workout though. Once you do that for a bit you'll naturally move further back off the saddle without having to put it down so much.

As for tyres, they are a personal choice, I've had tyres that I've thought are positively lethal on wet rocks/roots that other people think stick like glue! Don't be afraid to change them, maybe have a look at what other people are riding to get an idea what is used on your trails?

Keep at it, I find my progress on a bike is not linear, it tends to go in jumps, then plateaus again.