[@Skyrte]I [i]think[/i] that Tasuma's background works. Some of Regret's invasion forces had certainly been left behind after his carrier fled, and it was pretty much immediately afterwards that Truth sent his Jiralhanae-led fleet with orders to wipe out any remaining Sangheili and Human resistance. If Tasuma was among a small pocket of Sangheili-led holdouts left over from Regret's invasion force, I could see there being an impromptu truce, especially if a prideful Brute let it slip that the orders came from the Hierarchs. Miranda, Johnson, and the Arbiter returned from Installation 05 before the Fleet of Retribution, so they wouldn't have to wait for the fleet to get their truce "confirmed". 

I can tell by many of the little details in your CS that you've done your homework on the Lekgolo, so you might already be aware, but if you haven't already, you might try looking up voice lines from "Colony" in Halo Wars 2. Lekgolo are...weird, so that might help. At any rate, unless I've missed something, I'm pretty sure I can accept Tasuma. 

That does mean I won't be accepting any more Lekgolo for now.