Vandero

They could either convince more spren of the same type to appear in the physical realm, or undergo mitosis or budding, then grow with stormlight. The latter would be a mechanic as of yet not observed, but spren reproduction could very well be a thing, complementing the occasional spren splintering.

It was a chore to discover the aluminum alloy that counts as allomantic/cosmere aluminum, for manufacturing of guns. That means more than one alloy formula count as a viable metal or alloy. But different allomantically/cosmere viable metals have entirely different interactions with the magical systems. If duralumin just inherited aluminum properties, they'd have made duralumin guns, to begin with. It doesn't, just as bronze doesn't inherit the properties of copper or tin.

The feruchemical storing of a ship's weight is not something that was ever revealed. The lifeboat craft used an allomancy powered propeller, while the passenger's weight was stored feruchemically in varying degrees, modulating altitude. The bigger craft could very well work the same way.

I don't see it being against Taravangian's plans, although it could be hard for him to connect to Honor.

To remain Harmony the Vessel would have to give up affecting reality in meaningful ways. Sazed actively struggles against that. So instead of toning down each other the two component Shards will be able to act through Discord, being able to act, but the other half putting a twist in every action.