No you cannot use it as a mesh without a USG gateway and cloud key

Mesh yes, Wps no.

Mesh yes, WPS no.

The U6 could be used in Mesh. This should actually be supported by all Unify access points.

Please remember to use a PoE injector for the power supply.

WPS is not supported by Unify.

These are managed devices and not cheap stuff.

An alternative would be a U6 extender, which is plugged into the normal socket