Q:
Radiance and sharing...
A:
I use Radiance runes on my Aura-sharing Elementalist and all auras I generate myself and share with allies have the same duration.
I haven’t used it so maybe someone who has can correct me if I’m wrong, but the wording seems pretty clear to me. If you (i.e. the person with Superior Rune of Radiance) apply an aura its duration is increased. This would include auras applied to allies via the Powerful Aura trait.
If someone else who does not have Superior Rune of Radiance applies an aura to you then the duration will not be increased, though the #2 bonus should work.