looks like you're in good hands with Andrew here but just second what these guys have said these modules are so voltage sensitive, I would invest in a decent multi stage charger and keep that connected for these things also if you are ever connected to ista/programming keep it hooked to a charger.

Not sure if it has been suggested and might not be related because you are getting intermittent start but no fire but continuous crank could be a DEM/EWS misalignment. You mention you already have ISTA so maybe worth giving that a try, there are plenty of vids on youtube to show you how to carry out a DME EWS alignment with ISTA