>a "Build Failed".
Any related error(s)? Builds fail for any number of reasons, so...
Have you consider one project with multiple targets, that all share a common LS.sb?
Another option is to create a base project, using it as a template for others. Duplicate the original project folder, tweak paths accordingly, and make sure each project app is unique where it needs to be in Xcode's mind.
Also, just for grins, create a new project, wiping it's LS.sb, then option-clean, then try a select-drag/drop of the desired LS.sb into the new one with both project open at the same time.
As for XML copy/paste, keep in mind that the file in Xcode relies on specific line endings/wraps/returns, so that can be one gotcha, at least - be -very- careful when throwing that XML around manually.
As noted, I'd try an option-clean build folder...hold the option key, then use the Product menu, selecting to 'clean build folder'.