The Robe of the Whistling Fists is confirmed to be clickable from inventory and clickable as a hotkey.
As of November 2020, the following method works for quest component trade-in to Brother B.:
Sneak is a waste of time as Brother B sees through sneak.
Invis undead potions may simplify the process but they are not required for trade-in.
Assistance from an Enchanter or Bard may simplify the process but they are not required for trade-in.
Do all of the following to prepare for trade-in:
(1) If not done already, prepare 2 hotkeys (executable through your numeric keyboard keys): one hotkey for Feign Death and another hotkey for sit/stand.
(2) If not done already, place all 3 quest items in 1 bag and leave the bag open.
(3) Single-click 1 of the 3 quest items and leave the quest item on your cursor (mouse pointer).
<<Attention: Do all of the following as fast as possible before Brother B de-spawns>>
(4) Feign death facing Brother B. at his spawn point AND at trade-in distance.
Note: It does not matter if Brother B is facing your character or not.
(5) If feign death was NOT successful then repeat step 4. Otherwise continue to the next step.
(6) If Brother B attacked your character then wait for "Your enemies have forgotten you!" message.
(7) Wait for your character's feign death ability to return to an active and executable/triggerable state.
(8) With fingers hoving over the stand hotkey, mouse button left-click, and feign death hotkey do the following as fast as humanly possible:
(8a) Press sit/stand hotkey (to make your character stand).
(8b) Mouse button left-click (to open trade-in "Give" window).
(8c) Press Feign death hotkey (to drop /con to indifferent).
<<Attention: Take your time and do the following slowly and carefully>>
(9) If feign death was NOT successful then repeat steps 4 through 8.
(10) If Brother B attacked your character then repeat steps 4 through 8.
(11) If Brother B did NOT attack your character and your agro is clear, then continue to the next step.
(12) Add the full set of 3 quest items to the trade-in window and finish/finalize trade-in.
(13) Receive your new Robe of the Whistling Fists.
This is confirmed to work as of November 2020 and my character received the robe even while facing Brother B.
Edited, Nov 8th 2020 10:39pm by halfridge
Edited, Nov 8th 2020 10:40pm by halfridge