Use INSANE_SKIP_${PN} = "installed-vs-shipped" in your recipe to avoid this error. Orelse, add FILES_${PN} += "/usr/local/" in your recipe.
No.
*Only* use installed-vs-shipped if you're incapable of a) packaging the files or b) deleting the files in a do_install_append. Skipping that warning means you won't notice new files being added to the install that you don't package, which could result in broken packages.