Am I getting it right?
If I connect my solar panel (that gives max 6V at bright sunshine) to V-IN, that should cause no harm, yes ?
Solar Panel Input on Cube Cell Board
The schematic suggests that anything up to 7V is OK, that’s all I’m going on. The CE6260 data sheet suggests that its maximum input voltage is 6V. Either way, you should probably add something like a 1N5819 diode in there too to protect your solar panel (configured as per the one in there on the USB power line).
But note the previous post that advises that the node will reboot every time the voltage drops below 4.2V—the drop out voltage of the CE6260 regulator is 1V above the output voltage. That suggests that the power switching circuitry, while not revealed in the schematic, is not as sophisticated as that on boards that include explicit solar panel support.