Damn brother, are you sure about the first two points?
Reasonably sure.
1) For the Apostle Devourer
Female "Then, there is the Lord of Dark. God Slayer. Apostle Devourer. It is often those originating from the ninth, the tenth and especially the seventh, that worship it, though thankfully I'm not so foolish."
2) For the power ranking, while it's not explicitly stated, there is a certain structure to how Aglaecwif presents these beings.
Apostles are the monsterkind ancestors.
LotD, Ocean and Outer Twins are worshipped as gods, and she thinks that those with any sense worship the Outer Twins, the "true gods" of the bunch.
The oldest monsters that have been around long enough to remember, worship the Arbiters.
She seems to group the entities based on their power, from weakest to strongest. This, along with deadend3 which shows that entity (who is assumed to be LotD) annihilating Memory in an instant, makes it clear that there isn't any comparison between them.
3) As for if the LotD devoured the 7th, we can't really know what happened between them, but if any devouring happened, the reasonable bet is that LotD did it, having the title and all.