At one point in-comic, Jade says to "think of [the multiverse] like a giant solar system, but instead of planets revolving around the [Green S]un, there are many universes." We know the creation of the Green Sun is visible from the Veil in the trolls' session. Following this logic, would the Green Sun's creation be visible from any session's Veil, or only those orbiting it at a specific distance?

Should be only those orbiting it. I doubt sessions with a different Green Sun would have seen it.