As someone who used to race tri and now basically exclusively mt bikes, I would get a pivot to start.. IMO there is no reason to run a hardtail anymore. FS is way more fun, and I think faster as your descending will be way better.

The scalpel vs. 429 is a good question. I've owned s-works epic, spark 900RC, evil following, and currently santa cruz hightower LT and Blur.

Downsides to the epic and spark were tons of proprietary parts, not a fan of that stuff at all. Love the santa cruz stuff in that all parts/shocks etc are easily overhauled by my shop.
The new cannondale fork looks amazing, but if you need to send it away for any service, that's a huge pain in the ass.

I've demo'd the scalpel SI 2 last year, and liked how precise it was, but found descending tricky (and I can get to a bike park and hold my own for a day of lift access). However, the new scalpel has different geometry which should make it better.
Huge fan of everything santa cruz makes, lifetime warranty, great customer support!

Tires for rocky - I run on the hightower LT (6 inch front/rear) maxxis DHR II with ardents on the rear, I like rolling fast so give up a slight bit of grip but have found that combo works well for almost anything. Was just down in tuscon and had no issues with rocks.
On the blur I run maxxis ardent on front, icon on rear, when I race go to icons front/rear. I've tried some schwabe tires but generally haven't liked them as much as maxxis stuff.
I did get a 34 stepcast on the blur which I love, I'm only 175lbs but notice a huge difference and for the minimal weight difference would never go back to 32 mm stanchions!