You can adjust the left elbow location when using holdables. To do this go to the BP_SGKMasterCharacter and select either the ThirdPersonElbow or ThirdPersonElbowLeft (depending on if you are changing the left or right elbow location) in the viewport. Then set the Location and Rotation to 0,0,0 like the video below shows.
Next go to the holdable blueprint of the weapon that you want to change, so for example the BP_M4A4 and in the class defaults search for Elbow, then set either Elbow IK (Which is used for the right elbow) or Left Elbow IK (depending on which elbow you want to effect) to 0,0,0. Then in the class defaults search for Debug and tick on Debug Hand IK. Now save and close the blueprint.
Now open the BP_SGKMasterCharacter blueprint and go to the view port, then play in editor, pick up the and hold the weapon you are changing in game. You can now adjust the Ebow component in the BP_SGKMasterCharacter blueprint and it will update in game.
Once you find the location you want to keep for the weapon just copy the component location and paste it on to the Elbow IK (Which is used for the right elbow) or Left Elbow IK (depending on which elbow you want to effect) variable in the holdable blueprint, like the video below shows. Then save and remember to disable Debug Hand IK in the weapon holdable blueprint when you are done.