Also, regarding the Sun and Moon metaphors - yes they are found all over the Hindu scriptures.

Some shlokas use them for Ida-PingLA , and I think they are also used to represent mind(moon) and AtmA (Sun) perfectly.

However BG15.6 is clearly using them to demonstrate the enlightened state (light) versus ignorance (darkness) - as indicated in my first post. The enlightened state does not require illumination from a 3rd source of light such as Sun, Moon or Fire because in this state, in that dhAm (tad dhAm), the Being is Self-Illuminated/illuminating as there is complete absence of darkness i.e. ignorance - avidyA.

BhagavAn is implying that this place is Self-illuminating. There is no trace of darkness (avidyA) there. If there is no trace of avidyA , no desires, no specifically local-egoistic endeavours, no attempt to serve the ego, then there is no need to come back from that state.