Yes, stance affects your ability to butter. A wider stance is generally encouraged, but if it becomes too wide it impinges on knee mobility and your ability to shift your hips and centre of gravity back and forth. This ability to shift is equally if not more important to buttering than leverage. Leverage is more relevant for newbies cos they cant even press a board. But once you can press well and start doing butters into jumps/combos then a you will need to experiment with a happy medium.
In general, more length from your binding to your tip/tail means you have more wood to resist against the body weight you are applying and more distance to go when shifting your body weight past your bindings to get the board to bend. So in general, more wood, less flex.
Evo vs Sierrascope is preference. I find a reverse cambered board which is rockered in the middle easier to press than one which has an uplifted tip and tail. But you may not.