Hi SPANNER-DO,
I find your reasoning quite interesting!
Do you recall that there are, in fact, two pneumatic valves that have to be opened to supply air to the engine starter,
1/ the start control valve, the one you mentioned may malfunction, and
2/ the pylon valve, the one that opens to allow pressure to the start control valve, the one that will only allow reverse airflow, ie, towards the start control valve, when the FE selects that engine to 'ground start'.
Both close, at the same time, when the starter is released.
The isolation valves are not normally used here.
Actually, 4,1,2,3 is the best start sequence of all, taking system operation into account!
Cheers.