In episode 9, copy and paste from the folder that you already saved on your computer from when you cloned the whole library not from GitHub.
You should generally create a whole new project for each logger/program – just like in the Arduino IDE. The platformio.ini file must be in the top level directory for the project – not in a sub-directory. If you really want to have multiple loggers/programs in your project, each program must be in its own sub-folder and you must use the src_dir option within the platformio section of the platformio.ini file to specify which directory you wish to build from. The structure of the ini file is well documented: https://docs.platformio.org/en/latest/projectconf/section_platformio.html#src-dir
I was under the impression you already knew there was 2G available before buying the units. I don’t think it’s worth writing up a separate program just to check signal strength unless you’re planning to make multiple trips to the site, one to check signal and another to actually deploy the Mayfly. You’ll see within the first few minutes of running your logging program if you have signal based on the blink pattern of the modem and whether data appears online. If you find out at the last minute that you don’t have signal, just yank the modem off and deploy without it. Your logger board will be powered up ~5-10s extra every logging cycle trying to communicate with a modem that’s not there, but unless you’re in a really shady spot or not using solar, it’s not worth stressing over.