Xenolith

Force them to heal wounds.

Let's assume we're dealing with Radiants of the third Ideal, since with a few exceptions we have no idea what abilities more advanced Radiants gain. Because Surgebinders have Shardblades and can heal from normally fatal wounds, fighting in close combat is a really bad idea for Mistborn, even assuming that pewter enhancement is stronger than general stormlight enhancement. The question really becomes: Does the Mistborn have enough metal to drain the Radiant's stormlight from a distance, and what can the Radiant do about it? The surges that allow strong counterplay to a Mistborn trying to stay at range are Gravitation and Transportation. Illumination might also work for that purpose, but Mistborn have bronze. What we don't quite know yet is which surges allow ranged attacks, and to what extent they do, but let's assume Transformation works for indirect attacks at range, and that Dustbringers can attack at range as well. Then Stonewards seem to be able to reshape materials, and could use that to fashion a barricade and force a tie. So taking aside Dalinar, who may be as special as Vin, the Radiants most in trouble would be Bondsmiths, Truthwatchers and Edgedancers.

What I found interesting is that he got rid of his Breaths. That seems to imply he wants to Nahel bond it, which would prevent him from easily leaving Roshar. So he's planning on staying there for a while.

The problem with your theory is that the singers have had their ability to change form restored and Fused don't seem to be limited in the amount of Voidlight available. So Ba-Ado-Mishram must have already been freed. I'd posit that she is the spren of the Everstorm, and the Stormform listeners freed her at the end of WoR. Also, the thing they were looking for at the palace could just as well have been Elhokar's spren.

What exactly is complete symmetry? Different types of symmetry can very much be mutually exclusive. For example pentagonal symmetry and planar symmetry. (along the same axis)

I was thinking it could be "I will let others make their own choices." or some similar wording.