Thank you for sharing your data. So, if there is no series resistor, the Zener has little / no effect on the circuit until DC-IN goes over 6.1V. If you agree, we don’t even need the jumper - agree? If we can do without the series, resistor it opens up an interesting solution.
But first, this safety note: If we do away with the series resistor and someone connects a DC power supply that goes over 6V, it will look like a short from DC-IN to ground until something gives up the smoke (likely the zener). For this reason, I am tempted to pick a wattage for the Zener that is closer to 1.2A (I use a 3.5W 6v Voltaic panel).
OK, with that out of the way, I think we could support three different power use cases:
Battery - Only - perhaps with a big LiSOCL2 to enable an insanely long battery life (see my post on one year battery life).
Solar Panel and Battery - with the Zener diode protection in place.
5V / 1A DC Power Supply only.
Recall, the circuitry that enables the RTC to control the Enable pin is powered by the battery. To support the DC Power Supply only approach, I would need to add some circuitry to power this circuitry from either input (Li+ line in Sheet 3/3 in schematics above).
This is this a big deal? Well, I use a finite state machine approach to organize my code. There is an Error State and if the program gets there, I have a (small but growing) number of steps I will take to get things working again: soft reset, SLEEP_MODE_SOFTPOWEROFF, resetting the Particle session). I think it would be useful to add one more - remote power off. This would simply be setting an alarm for 10 seconds in the future and enabling power off sleep. This option is there in the current design but only if you have a battery connected.