All this planning about using low self-discharge LiSOCl2 batteries at VIN, and no one even once mentioned that the deep sleep current is 2-3 mA when a battery is connected at VIN!!
Please see this thread: Electron Deep Sleep current when powering from VIN without LiPo for the same.
We also had been thinking of using batteries at VIN and had designed a custom PCB for this, only to realize later that VIN had such a high quiescent current. That claimed 130 uA sleep current is only for the Li+ pin.
Particle should really make this thing more clear in their documentation.