Hi,
Sorry for the late reply.
I think your doubt is correct, if we set ignition source as power voltage, the immobilizer scenario will not work as expected.
If you can fix this in server level, that will be the best option.
You can create a logic in server which says RPM > 0 & Speed < 5 = Vehicle is idle.
If you can't do this in server,I have a complicated way in hardware level,if you need that let me know.
Thanks in advance
Naveen