For a simple fade-in and fade-out there’s no need to create a second storyline. Select your clip, press CTRL-V, click on the small icon on the right side of the compositing-bar and move the keyframe-handler to the right.
Best is to understand how FCP X is doing it’s Background-Thingie. Click on a Background-Template and choose “open a copy in Motion” Than you see, how this is done and can produce your own Backgrounds.
It’s one way to sort things out using Keywords. But when you look at the Event folder, every Keyword gets its own item. That way you ended up with trillions of single items. When using renamed Favorites you keep the whole thing a little more organized. By the way: why you can rename Favorites and can’t search for this names?
You can add a mask in the color correction tool. click on “affect outside” (not sure if this is the right translation) then lower the overall brightness to zero.