A lot of it is pure guesswork, but...

Other "reflections" are, indeed, alternate planes of existence that are likely similar to our own but due to the fracture at an unknown time in the distant past have radically different cultures and histories.

The "Source" is what would be termed the "prime" world in most other stories running on this sort of thing - in other words, the original world all the others are derived from.

Whether reflections suffer the effects of Umbral Eras is unknown.

We don't have exact details on what was happening on the First Reflection. Arbert mentions that it was being "erased" into "blank white perfection" - my personal interpretation of this is that it was being, well, literally erased from existence. Voidsent could not be spawned because there would be absolute nothingness. Not even the fragments of barely hospitable terrain we know to be in the void (World of Darkness) would exist. Even if a voidsent were spawned from such nothingness, it would exist in complete contradiction to those from the destroyed Thirteenth Reflection - start out enormously powerful, but have a very short "lifespan" and degrade into nothingness over time.

This came about because the Warriors of Darkness had banished all Darkness - all traces of Zodiark's influence - from their world. This tipped the cosmic scale too close to Light, causing a runaway effect that began to drain the aether from the world (instead of compressing it into a singularity as happened on the Thirteenth).