board length is based on weight mostly.
carving I would bend your knees and squat a bit deeper so you have better center of gravity.
although according to their website the Wrath for 2014 is a flat-rocker board. idk if it can affect carving.
I'd say it's probably not the board or bindings as I can carve just about any board with edges
Both of these. At 170lb you are on the upper end of the weight range for that board which can make it more prone to washing out. Additionally, a flat rocker profile gives the board a "looser" feel. (looking at specks for a 2014 154 Wrath)
Does that mean you need a new board? No, its just more important that you use proper form now to hold an edge. Had a ~170lb guy and ~155lb guy riding 130something Burton Minis at the mountain yesterday, so you're not bad off.
As for your stance, that is just something you will have to experiment with until you find what works for you.