I can help with some of these questions!
First, I don't believe you get a steam key for the dlc if you download here on itch.io. So, if you prefer playing on android I'd buy them here since you can't download the android files on Steam.
Second, on the dlc download page, there is instructions for how to download the dlc for android (and other devices). From what i've seen for android, it is automatically updated once you install the .apk file to your device. So, you don't have to put it in any folders, but make sure you install it to your device.
For PC, you do have to manually put the rpa files (the files you download) into the 'game' folder for Our Life. It's really simple, I promise! And once you open the game, you'll have a little icon on the top that you can click and make sure your dlc was installed!
but also, you can just buy the dlc here on itch.io, so you have access to android and PC file versions, and apply it to your Steam base game version for Our Life, since the files are the same! I think it's a bit different to locate the 'game' folder to put the dlc inside on Steam but it still should be pretty simple.
can't answer about the plushie, but let me know if you have any questions :))