If there is enough precipitation either observed or in the forecast for that day, the flex will not run. Most of the time this extra precipitation will keep the soil moisture level from going below zero.
An IFTTT recipe probably won't help in this case. I would see how the Iro behaves and decide if you want to set a manual rain delay. If there isn't enough rain to delay the flex and the moisture levels are close to zero it might end up running.
Yes, the next day it will look at the observation (versus the forecast) and make any corrections.
This Spring we will be releasing an overhaul of schedules making them simpler and combining the best of fixed and flex schedules.