Comment 10 for bug 1825766

On 1): The metainfo/appdata file is only enough if it is complete, so, contains an icon reference and name/summary/etc. tags. If it doesn't, then you'll need the .desktop file as well and a launchable entry.

On 2): All of these locations are wrong ^^ Metainfo files *must* be installed into /usr/share/metainfo in order to be valid. Refer to the AppStream specification (specifically https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#spec-component-location) for details.
There's also an online tool to help people making metainfo files at https://www.freedesktop.org/software/appstream/metainfocreator/#/ now

As for that existing "appdata" file, make sure it's actually an AppStream metainfo file before installing it somewhere else - maybe it just coincidentally has the same name as a metainfo file, but actually is something different.

Also, google.chrome and network-manager need their metainfo locations fixed ;-) It's likely that support for the legacy location will go away soonish (almost nothing installs files there nowadays).