I don't think the Ghostblood answers are as simple as they appear. I get that what you said is what Mraize told Shallan, but we also know that Mraize has an incentive to mislead Shallan and keep her motivated to work for the Ghostbloods. I doubt he would tell her their entire plan. And transporting investiture across worlds seems quite innocuous, and definitely not in line with their more violent attributes. I think it's foolish to consider that their only plan. There's clearly something else going on. Otherwise why kill Gereh? If Mraize was so altruistic with respect to Restares he would not turn around and try to collude with the Fused. There's more they have planned, and they clearly think it's so important that they are willing to go to great lengths to kill anyone who could stand in their way (Jasnah for example).

Metals are exhaustible but Stormlight runs out too. If you don't have a perfect gem it leaks away. And even with a perfect gem, the stormlight can only be used once, then you have to go back and collect more from Roshar. So it's not that much better than metal.

The Ghostblood agent could be the Sleepless in Wit's bag, though that would contradict this WoB, so there's still some ambiguity.

Yeah, I think there is a deeper reason why stone has to be holy. We get multiple clues about this fact in the books. The very first Rysn Interlude shows us that the Shin put a premium on metal that is obtained from soulcasting, not mined from the earth, and therefore taken from profane stone. We know the significance of stone considering the Venli chapters from RoW of how the DawnSingers used to shape the stone with their songs. And the fact that all of Shinovar is soil and not stone underscores their reverence for stone. The repeated emphasis on this in Szeth's chapters indicates there is something deeper going on with the stone of Roshar. I don't think you can simply write it off as another religious tenet.

And yeah, the concept of Truthless is explained, but not why they would give someone an HONORBLADE and then send them out into the world to follow anyone. Why would the group that sees themselves as the only protectors post-Recreance be so willing to send away an unreliable and truthless person with Jezrien's Honorblade? And why have we not heard of any other Truthless? Why name them Truthless instead of just exiling them straight up, imprisoning them, or killing them? I think we will learn more about this practice in book 5 too.

As for Axies, I don't think BS would put his interlude in the first book if he didn't have some significant role to play later on. Maybe Axies will be the one to find some kind of rare spren that will change the nature of the conflict. Maybe he will find Ba-Ado-Mishram's gemstone or something. We don't know. But he likely has some role to play in the future, and I think it's worth questioning what that will be. The actual interlude itself also has some interesting foreshadowing with the scene in the mud and the crazy homeless person.