1. Microchip (PIC) has been THE supplier for the automotive market. The PIC controllers we all know basically all come from this root.

2. There is no standard, but taking the application into account one would expect an 8-bit local bus to be sufficient. The PIC family of processors, with some "modern" exceptions, are mostly 8 bit as well, so that supports my statement.

3. As far as I know the standard industry rules and regulations apply, with an exception here and there. In the time that EMC requirements for industry were limited to 10V/m field strength, 30V/m was applicable for in-car safety systems (in Europe) like the air-bag control. For motor management systems, I don't really believe that there are any regulations. You may have heard of the control system problems BMW encountered with the introduction of their latest 7-series: it has taken them almost a year to solve only the worst problems, which included full engine stops whenever and where-ever one would not expect it.

4. Would there be a reason to use an RTOS? I think much depends upon the layout and size of the system. For smaller cars, the control system may only be there to control the engine and some additional gadgets here and there. On the more expensive cars, there is much more to control, like in the BMW 7-series mentioned earlier, there are over 100 sensors and some 30 servo motors installed that require the attention of the system. The complete control system is spread over a number of networked nodes. If an RTOS would be necessary, I guess you would find it there.