For a philosophical parallel, a broad solution may be found in https://en.wikipedia.org/wiki/David_Lewis_(philosopher) style of realism.
This allows for isolation and for similars in other worlds (other computers) as “counterparts” but without message passing.
Lack of message passing between worlds may be an extreme for security but framing a “perfect secure computer” can start with David Lewis as a starting point.
From that, you can build message passing that violates David Lewis.
Really real. Not abstract. This doesn’t mean physics though but in a philosophical sense.
So, if there’s a what-if, treat it as actual but happening in an isolated Universe.
Benefits are numerous to this approach.