They could have used RTEMS (was designed for missiles) - on some level, if you design accordingly, you dont need a realtime operating system anymore - you can get by without it, and its easier to find people to design software in my opinion for not an RTOS
The operating system provides a standardized IO API and schedule processes/threads. If you don't have fancy IO (LCD, hard drive, network, etc...) and multi processing, there is no need for an OS.
Sorry, I don't understand your point at all. Even fairly simple looking applications can end up doing all kinds of things at once / in an interleaved manner. Preemptive scheduling is simpler and therefore safer than trying to squeeze everything into some kind of gigantic global state machine.
