Trusk'our

This is probably within the realm of reason, but you would need to use an infusion of extra Investiture, and it needs to be Unkeyed if you're using it to fuel a different system as far as I'm aware.
For example, you could make a Mistborn via Forgery, but you'd need a lot of extra Investiture.
Hope this helps!

Good questions. 
We don't know a lot about Trueselves or the extent of their power yet, but we do know that simply blanking Identity and healing won't typically change how the healing works, as self-perception will still be there.
As for storing only specific portions of Identity, maybe.
It probably depends on whether an Identity can be broken down into smaller, discrete Feruchemical attributes (like senses or Connections) and how skilled the Trueself is.
Personally, I'd guess that it is possible, but would take practice and a knowledge of what they were trying to do Realmatically. 

I would venture to guess that the loss of energy is because the metal isn't getting converted into energy, as a Harmonium/Trellium bomb works, I think it's just trading one thing for another. Matter for Investiture, which isn't necessarily one-to-one.
Though as @king of nowhere said, both the Investiture and the metal return to Preservation's system eventually.
It's weird. Allomancy does rely on a small seed of innate Investiture to function, but also metabolizes a metal to draw more fuel directly from the Shard of Preservation.
Feruchemy is the art that really relies purely on your own Innate Investiture, though even it breaks the rules a little bit and draws extra power from the SR to facilitate change in states of matter. 
There's also just the fact that the term end-positive, end-neutral, and end-negative are all imperfect terms that kind of break down the more you mull it over.