You omitted the "or" at the end of (1). Gravity is the "means to prevent introducing air into the [fuel] system" of an aircraft with the tanks higher than the fuel selector/EDP etc. That works, even if the fuel selector is able to select more than one of those tanks, simultaneously.
All fuel systems will eventually introduce air into the system, through exhaustion or starvation. That's not the point of 23.951.
The point of 23.951 is system design. An aircraft with the fuel tanks lower than a fuel selector that can select more than one of those tanks simultaneously is not designed in accordance with 23.951, unless there's some further component to overcome the laws of physics demonstrated in the video I posted and which 43 explained. Given the ATSB report I posted earlier, it seems to me the Commander 114 has a basic design flaw in the fuel system. I'd never fly one with the fuel selector in "BOTH".