So you're saying that Nicrosilminds might hold extraordinarily specific investiture, like how Tin works?
Wow. That would be so OP, it's not even funny, but I like it. Possibly just because it's new and makes my mind turn with the possibilities, but whatever.
For this to work, the power of a Feruchemist would have to be not the power of storage and return, but the power of conversion. They would be constantly gaining a specific amount of all attributes specified in Feruchemical documents we have access to (excepting a few, such as youth, which they would have a specific amount of when they are born, and they spend it as they go through life), and they can choose to divert-and-convert said energy into specific Investiture in a metal that can be drawn upon later. Nicrosil would then be the "Store-all," but the energy they get would not be "investiture" until they make it so.
It is generally considered plausible that if one were to store an attribute in a metalmind whilst storing their entire identity into an aluminummind, another Feruchemist can access it. If a Feruchemist, for whatever reason, tapped-and-stored something into Nicrosil, such as an important memory, while at ID=0, and then left it in a safe place, a Nicrosil FERRING would have access to the memory.
Also, what if a Nicrosil Ferring were to become a Knight Radiant? One limitation we know about Stormlight is that it is easy to infuse gems with, but storage really stinks because it's not really locked in there. However, Feruchemical storages never decay. If that ferring were to infuse themselves, then store it away in their Nicrosilmind, Poof, that limitation goes away, and we no longer need to carry tons of money in our pockets! And, that's not even counting a Nicrosil compounder.
As to the Breath thing, I think you'd just end up with either one REALLY strong Breath, or TONS of little tiny wimpy Breaths that would together be greater than your original storage.