Maybe Gavilar thought that *Unit Them* was meant for the Heralds, thus going to Braize as a rescue mission to retrieve Taln. If Taln was rescued, before he broke, then maybe they would have had time to prepare... (Gavilar's potential reasoning) Maybe Gavilar saw how the Heralds were in mental distress, present on Roshar but lacking purpose in their duty to prepare for the next desolation. Quick fix? Bring back Taln to try ending their guilt, and get them motivated? This could make sense in my mind, as in, Why would the Heralds discuss, with Gavilar, about going back to Braize, otherwise? Also, about the trapped *voidish?* spren, this could be another argument on Gavilar's part: "See guys, nothing to be afraid of, we can trap Odium's spren, if they were to chase us there.". Sorry if this idea was already brought up, I didn't have time to read through the entire post.
Edit: also maybe taking Odium's Perpendicularity, warping it to do their biding, using the trapped spren somehow, or other means?