How best to work with landscape splines and world composition?

I am banging my head against a wall trying to use landscape splines with world composition. I see no documentation or tutorials anywhere that explain the process.
When I create different landscapes using world composition, adding landscape splines across the different landscapes will redraw the level bounds and then interfere with level streaming the landscapes. Sometimes it will load in a landscape that is far away from the character where the landscape spline reaches to, but will have a level in between the character level and the distant landscape that is unload.
I’ve been adding the spline to the persistent level but I’m not sure it matters since I’m not sure if there’s a way to designate that the spline belongs to any particular level.
I’ve also had where adding the spline makes it difficult to add a new adjacent landscape in world composition that lines up to the selected landscape making and even when I move the landscape into place, it doesn’t allow me to sculpt across the two.
How can I use landscape splines with World Composition so that it doesn’t redraw some of the current levels bounds? Or is there a way to create landscape splines within their own level?

You should really separate your questions into clear parts instead of having them grouped up in one paragraph.

When I create different landscapes using world composition, adding landscape splines across the different landscapes will redraw the level bounds and then interfere with level streaming the landscapes.
A: Go to World Outliner, click on Level Bounds, go to Details and uncheck “Auto Update Bounds”

I’ve been adding the spline to the persistent level but I’m not sure it matters since I’m not sure if there’s a way to designate that the spline belongs to any particular level.
A: Change the active level to the level you want the spline in, then draw the spline across the landscape.