I think that it does store your abilities to store your abilities. You are just still able to stop filling it or else you'll just enter a negative feedback loop and suddenly lose all your investiture, which would be sad. (which actually now that I think about it would be a powerful weapon, if you could trigger it on someone else)
According to the talk I've heard around F-nicrosil, yes it would boost allomancy almost like a controllable duralumin, like @Quantus said
I think this depends on the general feeling people see investiture. If the world thinks that they are separate, then like tin you would need a metalmind for each. If they don't it would be more like a coppermind. This is because, from my understanding, the reason tinminds need to be seperate is because each sense is viewed as separate, while your memories are more of a hodgepoge mixture.
as for your last question, I don't think we have an answer, cause none of this is cannon (which I actually didn't know until I read this thread!)
Hope this helps!