After working this out mentally and doing some additional research on Arduino interrupt handling I've learned that accessing time functions inside an ISR appears to be a bit of a tangled subject. This would make it quite a bit "hairier" to detect sequence with only the low-res signal.

This got me wondering how the ECM behaves. If no-one knows I'll confirm myself when I have a chance, but I'm wondering if it will start after:

1) disconnecting the high-res signal
2) removing power from the ECM to clear learned cam position

I know it will start and run without the high res signal, but it was more than a few years back when I worked on replacing my opti sensor and I don't know if I ever disconnected the battery throughout that process. I don't believe I did.

If the ECM is improperly sequencing the injectors I would presume the engine may run, but poorly. If, on the other hand the ECM simply won't detect cam position or fire the injectors without the high res signal that simplifies things immensely. Detecting the position becomes considerably simpler and faster if we need only count high res pulses to determine a trigger slot's opening size.