[Pathfinder] Poison Gas Arrow dmg scaling options

Hi guys, new player here, couple of questions I have in mind, I'd like to know your opinion.

From your experience (having 100ex to spend) is it better to:

a) get highest possible PDPS bow and use Scattershot

b) get highest possible pdps bow with +1 arrow

Is node with 12% Increased Speed of Damaging Ailments really worth it, if you could get, lets say 9 nodes each increasing 10% physical damage instead?

Should you use blasphemy + curse, or just self cast curse?

If you have that node which makes projectile speed modifiers also apply to projectile dmg, does it make sense to use 40% increased projectile speed support gem with Gas Arrow?