Does Righteous Verdict stack with Whispers of the Nathrezim?
Yes. It is multiplicative.

Should I delay Avenging Wrath/Crusade for BL/Hero?
Possibly. If delaying Crusade causes you to miss an opportunity to cast an additional Crusade it is not worth it. Go reread the Avenging Wrath/Crusade section.

What is a good Pawn string for Ret?
Don’t use Pawn. Sim gear. Don’t make excuses saying you can’t sim during raid. Sim before raid and just adjust the ilvl of the item to see what the minimum required for it to be an upgrade is.

But “xyz” said 25% haste!
That isn’t a question. Also. NOOOOOO.

Can you include encounter specific tips?
Starting in BfA you should expect more active work on encounter specific tips.

So, what is the minimum amount of haste I should have?
There is no minimum nor maximum haste.