Some of the Elites would have to speak to that, but my guess is that external watchdog would be the only way to approach 100%. But for me, it's [Manual/Threading] worked for a few projects that were extremely sensitive to wasting precious battery (primary, no recharging available). But then again, I don't have 50 Electrons running Manual/Threaded for a decent sample size.
I don't know what all your code does, but what impact would using Manual/Threading, but No timers, No ApplicationWatchdog, no external watchdog have? It would just be a 1 hour deep sleep, wake up, allow up-to 3 minutes for a successful connection, then go back to deep sleep for 1 hour no matter what (the 1-shot approach)? You could throw in a system.reset every 24 hours or once a week.
But there will always be a tiny chance that the cell modem can get stuck in a funny state and continue to waste power. H/W watchdog would be the best thing that I can think of to mitigate that, but you will need to recognize that situation first (I'm not sure how to).