No problem! You can get both of the 18+ bonus moments on the $10 tier. It's totally fine to join for one month, get the perks you want, and then cancel before you're charged again. If you bookmark the download page for the bonus moments you'll be able to go back to that page and download them again even if you cancel the subscription. But don't bookmark the post on Patreon talking about the content, because that will be locked for you once your subscription ends.
To play the bonus moments you download them and if you're on PC/Mac you have to open the folder like you for for Our Life on Itch. The bonus story are totally separate builds that you play on their own. They aren't added into the main game like a DLC. I hope that helps!