Exclude certain entry types from Entry URL format?
I don't know of good, non-hacky way to do what you're doing.  Every entry in a structure should have some meaning otherwise you're going to fight the system.

Is there a reason why those top level landingPageGroups are just "vestigial" and serve no purpose? That would make them good candidates to break into separate sections. So Major Campaigns would be a new structure section, Major Events would be another new structure section, etc. 

You could probably hack your web server to rewrite slugs so that /major-campaigns/moving-office becomes /moving-office but for every section you add, that becomes a nightmare.

Even if you think you'll use one template across the board, it's inevitable that at some point you'll be using different templates for each "section" as well. I don't know your content strategy but if you only use one section, your entry types will likely multiply too.