2D
It sounds like the current version of the (Honeywell) KLN94B manual has been dumbed down relative to the Bendix King one you have. You are probably right in that if receiving >3 satellites, a RAIM error is unlikely to get flagged. Which is probably why bose-x, never having set the QNH, has never seen anything untoward, because most of the time you receive at least 4 satellites (usually >6).
What I don't agree with is where the displayed altitude comes from. It comes from the GPS, purely and entirely. The altitude encoder output is not displayed directly.
I can't speak for the GNS430 but I am told it works the same way.