Whirlpool on BH is not good imo because it’s not like you want to spam BH. Only every 8s or so for the buff. Otherwise you lose channeling time
Whirlpool on Blade Spirits is not bad because if separate Spirits proc it, then the Frostburn from Whirlpool add up. Which is not the case when you proc as a player.
Dunno how many Blade Spirits you have there but 2 of them can’t maintain Murmur full time on a single target.
Yeah, as I said I don’t think it’s a good idea
Remember that you have 2 chances with DW Shadow Strike so if it displays i.e. 70% it’s actually 1 - (1-0.7)^2
You may not know that but I actually had DW Chillwhisper Reaper build on the forum. Imo it was optimal for a short time, before the game changed a lot , DE was buffed massively, Phys res disappeared etc. I had 43% - 67% Phys res on it depending on variant 
Which was actually needed for Calla then which DE way lower damage.