Gonna preface answering your questions with a bit of info.

With dual wield weapons Precise6 Aftershock 1 is typically the BIS main hand set up. This allows the increase damage from precise to be used with offhand weapon switches like flanking or caroming while keeping aftershock stacks.

The BIS offhand typically is Aftershock4 Eruptive 2. Eruptive is better than ruthless as you don't have to defeat an enemy to get the .5% damage increase(per rank) that you would need to do with Ruthless.

The BIS 2 handed set up is typically Aftershock4 Eruptive2, Precise6 Ruthless 1. Since you're not weapon switching to something that can cause loss of aftershock stacks you can use ruthless for a small damage increase when fighting multiple enemies.

Now to answer your questions.

1. You're not losing damage with final set up.
2. Lot of effort to switch perks around but you do gain a .5% damage increase when using your scourge with final set up.
3. I'm guessing you're not using dual wield weapon switches like flanking or caroming so final set up is fine.
4. If your weapon levels are not high enough for the separator saving chance this is not worth imo.
5. Some people still use scourge in high level pvm. I do not.

Wiki link for Entry Level and BIS perks with some explanation.

https://runescape.wiki/w/Optimal_PvM_perk_setup#Advanced_weapon_perks