My initial thought was that the 4th just changed MC's memory and made him think that she turned into a hot chick and blew him. But after killing Jake, it's obvious that the 4th can also shapeshift, since she takes a snakelike form to constrict Authority.
Deadend 3 also gives a clue, assuming that the darkness monster is indeed the 7th, that he has some light based power? The one that ultimately kills MC.So I think it's fair to assume that the numbered have both traits to a certain extend.
DrakoGhoul's theory of "cross" inheritance fits these clues,
I think that it's more complex than that and the theory needs to take into account the differences between human and monster physiology. Perhaps in time, through Deryl, we will learn more on the topic.
I do have an uneedlesssly complex theory on the matter, but I don't want to talk about it yet cause I haven't figured it out completely. However for that specific part it's fairly straightforward, even if I'm making some big assumptions.
1) We know that the Arbiter can "possess" the MC.
2) We also know that the Apostles, just like the Arbiter, can't manifest in the "human" plane.
3) We know that Apostles created monsterkind.
4) Lastly we know that Apostles use Avatars to do their business in the material plane, exactly like the Arbiter does with MC.
With these 4 clues, I think that the Avatars are monsters, and they are the children of the Apostles. Essentially the twins fucked each other or used whatever breeding method they have iat their disposal, to create monsters that they possessed.
Due to that, the Avatars do indeed have both powers, but that doesn't necessarily mean that the Apostles themselves have both powers.
However, since these avatars carry both traits, it's possible to pass either or both, depending on the host's physiology.