I think some confusion about the loading screen arises from the question: what do you show when you are loading the loading screen?
If you use a loader layout, Construct still shows the default loader while the loader layout itself is loading. This is just on the off-chance the user is on a very slow connection or for some reason your loader layout uses a lot of resources. If your loader layout is small and simple, it should load quickly - so you can set the default loader to "none" and you will only ever see your custom loader layout.
Even if we supported some custom splash image for before the loader layout itself is loaded, what do you show while that is loading? At some point you just have to either show some pre-set message or image - which cannot be customised, because nothing is loaded yet, not even the settings for the game - or show nothing. You can already do both by using either Construct's default loader, or turning it off and showing nothing until the custom loader layout appears.
If you have a small, simple custom loader layout, and you turn off the default loader, then you have basically arrived at the same thing as a splash image or custom spinner.