Hi @GodlessGlen, thanks for taking the time to post :). I can completely understand. There is a learning curve especially when one is used to other methods of visual layout or has learned CSS using different tools/methods. My advice is to keep with it, there are things to learn in a new way in Webflow, and it may seem daunting when starting out. I totally agree and we are working to improve this area.
I noticed you had another post and have been making some changes, are you still experiencing any issues?
Regarding the background image, the interaction only works on one main element at a time. To change the opacity for multiple elements, create an interaction with multiple trigger steps, with each step targeting a different element. Those triggers will execute one right after another, allowing you to create simple or complex interactions.
If you put other elements inside a container, and set the parent container opacity, the opacity will be set for all elements inside that container, so you might look at using an Image widget instead of a background image, and target that image element by class name separately than trying to set the opacity on the element having the background image.