Golden Arrow is an arrow that automatically hits its target.
When volley archers use it, with extended augment, one use turns every arrow in ONE attack into golden arrows. This does not include rapid shooting. So ONE attack, with multiple arrows from multishot being turned into golden arrows for automatic hits if you use multishot (which you WILL do, as that's the entire purpose of extended augment), and that is it.
Also, you cannot target multiple enemies with multishot, all your arrows are on one guy. As for using golden arrow on a multishot rain of arrows, it only means you don't have to roll to hit. Everyone automatically gets hit with rain of arrows, with no save, although improved evasion technically automatically makes the damage half. But only twice per day, and that's at 18th level.

As for sniper archer...my god, it IS complete cheese. I mean, you're going to be hiding most of the time, so you only have a -10 to hit for a coup de grace since you automatically critical when hidden. I should probably just nerf it to forced critical but no forced coup de grace.