I don’t know what that error is =( Usually if I got something like that I would manually wipe the Engine/Intermediate and GameName/Intermediate, the generate project files again.
DFAO page is here - note the documentation does not contain DFAO improvements since 4.6 like it working on instanced meshes or landscape
First google result for DFAO btw =)
No, DFGI needs some work before enabling by default, so you will have to opt-in for 4.8.