Is a Mithril Heavy shield considered "light weapon" for purposes of off-handedness (i.e. -2/-2 instead of -4/-4), with a one-handed weapon in your other hand?
I would say so no. As the shield is now a"weapon" for the shield bash and it states a weapon does not change size category or the ease of use{weather it is light ,one handed or two handed}
So no by RAW I would say it does not make it a light weapon
Agreed. A longsword isn't made light when it's mithral, so neither would the weapon-properties of the shield.
A large awkward object stays large and awkward, even if it's feather-light.