While we’re wishlisting, I don’t think Living Metal should make the unit tougher—quite the opposite. It should be weaker, fluff-wise, with rapid regenerative properties.
Balancing this is tricky in an alpha strike meta, but that’s less important than fluff, honestly.
How does that fit with the fluff? Living Metal can act to absorb the power of an attack, it's like a ballistics vest being hard yet pliable, instead of a piece of piece of glass that is hard but brittle. I'd also hate for more of Necrons power budget to be allotted to healing vehicles, 1 wound a turn is awesome, it's fluffy it has an impact in some games but it isn't the main thing Necron vehicles have going for them, they also have high T or QS or they are flyers, they have shooting or whatever.
The “living” part of the “Living Metal” moniker, plus the ubiquitous presence of minute scarabs festooning large models like the Tesseract Vault, plus the existing renegerative in-game abilities taken together suggest to me their conception is more of a living skin of metal (ex. Avengers Iron Man nanite armor). It’s not impossible that they consider it to be what you describe, and indeed, Hardness v. Toughness is absolutely a real physical phenomenon that could be used to describe Flesh v. Metal.
But that would further push me toward expecting that Living Metal equipped models are expected to be less tough and more pliable anyway? Taken together with the “living” part of the name and it definitely suggests regeneration.
I absolutely agree that for Power considerations, the whole Regeneration/Reanimation theme for Necrons is not a recipe for balance success (it’s either too good or terrible)—but that’s not as important as theme coherence when designing a game (though strict thematic adherence is also not a good recipe: exceptions can frequently be used to prove the rule when used correctly).