(perhaps) the ethanol is a contributor to the nylon grommets failing - as a root cause. And as you speculate, that introduces moisture leading to the eventual issue with the solder joints.
FWIW: I have one injector (old style but as new) that was installed about 30 months ago along with the switched power mod. Other 5 injectors were done about a year later -- also using switched power. It will be interesting to see how long the 30 month injector goes before it runs into issues? At least I'm might have some idea about when the other 5 will start to go???