I like the small jig part.
But I don't understand why pin 9 shouldn't be soldered. Isn't it pin 9 and 10 together? As it is I thought only the metal part mattered, not the whole joycon.
I don't know if the purpose of pin 9 isn't documented, but it is not actually ground — it just happens to be set to logic low when a Joy-Con is attached, and logic low is referenced to ground. This means that it usually works. But at best it's not guaranteed, and at worst it simply won't work.
Pin 9 is connected to an output on the SoC, and connecting it with pin 10 could be unreliable at best (especially if the rest of the joy-con isn't there to trigger the Switch into setting it low), potentially dangerous at worst. If for some reason it's set to logic high, for instance, that could harm the Switch. And either way, as I said without the PCB from the Joy-Con there to tell the Switch to set pin 9 to low, it probably won't work at all.
You want to connect pin 10 to ground, which will then bring
that to "logic low" safely. See the following:
https://reswitched.tech/hardware/joycon#joy-con_connector_pinout (note that this page is out of date and doesn't mention that pin 9 is
not a safe ground output, nor does it mention that pin 10 is used to enter RCM mode)
So if you solder pin 10 to either of pins 1, 2, or 7, you'll be safely pulling it low without any risk of harm to the Switch's SoC. If you solder it to pin 9, it's not guaranteed that it'll even work.
