Need advice on 2009 Nytro suspension adjustment

I would start with your torsion spring first. Change one thing at a time so you can isolate the issue. If you can't get it soft enough, you can probably order a softer torsion spring online. Sucking up your limiter strap is going to increase your ski pressure and make it a little harder to steer though, so be wary of that.

If you are still fishtailing and need more sled weight on the rear, you can move the rear suspension up in the chassis, which effectively lowers the back of your sled and will shift the center of balance more towards the rear.

Watch this video, it will explain how you should setup and balance out your sled as much as you can:

https://www.youtube.com/watch?v=O8FFQHetosw