To address your points:
1. Any power source connected to the 2 pins labeled “Ext 12v” will power the Mayfly but will not charge any LiPo batteries connected to the two LiPo jacks.
2. The solar panel connected to the JST jack labeled “SOLAR” should never exceed 6 volts. The charging circuity on the Mayfly uses a small chip that has a 6v maximum input (and 500ma max charge rate). This is the reason that connecting a 12v source to the external power pins (see point 1 above) won’t charge the battery. The 20v-1A note on the schematic denotes the rating of the protection diode, not the input voltage source
3. Using a 12v-to-5v USB converter connected to the Mayfly’s USB jack will power the board, and charge any LiPo that’s connected to it, so that’s the preferred method for now. Hopefully the next version of the Mayfly will be more flexible about input voltages and charging.
A couple other points to make:
If you’ve jumpered over SJ13 to power the bee socket’s Vcc pin (pin 1), remember that the bee socket’s pin will be continuously powered by whatever battery is on the LiPo jack, and will not turn off even if you turn off the Mayfly’s main power switch. If you have a board like the new Digi 4GLTE board, the 4GLTE board will wake up when the Mayfly is turned off since the Mayfly can’t assert the sleep pin anymore. I’ve modified a few Mayfly boards to get around this issue, but it’s not easy, and have now made a tiny new adapter board that goes between the 4GLTE bee and the Mayfly that will solve this problem, along with giving the user some handy status LEDs since the 4GLTE bees have no indicator lights on them at all. I’m hoping to have those adapters in about a month.
Second, the CS451 needs 6-18v excitation, which the Mayfly can’t provide. How are you powering the sensors, and are you using the SDI-12 or RS232 protocol on them, and how are you interfacing them with the Mayfly? Without an adapter or voltage level shifting circuitry, the higher voltage signal will damage the input of the Mayfly. I’ve made a variety of custom level shifters and voltage boost boards for different sensors (6v, 9v, 12v, etc) or used relays to control power to higher loads. Either way, the signal needs to be conditioned properly before feeding it back to the Mayfly.