It mentions that Occipitus was once part of Celestia; but as what?
An 8th layer, or just a chunk of one of the 7 layers?
I agree; it's just a chunk that broke off.
I interpreted the book to say that the plane of Occipitus had always existed as a layer of the Abyss, but that a chunk of Celestia (not sure what layer) had been torn off and cast into the Abyss. This occurred when Adimarchus' demonic army managed to invade and capture a portion of Celestia.
Centuries ago, a demon army invaded Celestia, intent on rampaging and pillaging as much as they could. The heavenly host repulsed the demonic army, but at great cost: the angels of Celestia had to cast the part of Celestia occupied by the demons into the Abyss, tearing apart the fabric of the plane. This massive chunk of planar matter came to rest on the 507th layer of the Abyss, known as Occipitus in its denizens.
It doesn't say what specific layer it is from. However, since the Seven Mounting Heavens of Celestia get progressively more good as one climbs from the first layer to the seventh (in fact, there are no accurate accounts of the seventh layer as no one has ever returned from it), I would guess that the demons only made it as far as Lunia (the first layer) or perhaps Mercuria (the second).