While your entire post was good and something I´ve pondered myself I zeroed in on the question of the third/fourth specifically. It seems to me that while the rest of the apostles seem have names or portfolios that has concepts that are fairly broad the 3rd/4th do not really. To me they almost seem "lame" in comparison. So I wonder if their names are really only a way to make humans make sense and simplify a much broader concept. After all monsterkind and their "nobility" if you will do not use these names as far as I can tell. Its always "the third/fourth" when they are adressed.
To start with the third its portfolio is very likely some sort of variant of matter and how to change it from one form to the other. On most creatures the easiest way to start out with the ability is to change around yourself. It resists the least and you have an instinctive knowledge on your own matter and status. To understand things well enough that you can change something that isnt you requires much more practise and power that comes in later through evolutions and training. Hence why the trait and the apostle is referred to as "the Body". Its simply the most visible and early way to understand it. And while laws of physics are more of a loose recommendation in later evolutions I´m thinking that the "Law of Mass" applies early on so its easier to use your own body to make changes within a relatively close span of its size. An early example of this could be when the MC tried to make himself bigger and he failed when going to far out of his usualy size.
The fourth is a more tricky thing but to me I reckon that what it represents is not so much memories but rather "matter state" or something like that. (using that word to respresent the structure and form/energy matter is at a specific point in time) I´m no physicist so my speculation could be very off here but it seems to me that memory is simply a frozen earlier timeline matterstate that gets "stored" in some way that we recognize as memories. To me this sort of tracks in the way that early on we could only read the matterstate of present time organic creatures. Trying something even inorganic was much harder and could only be done in very small amounts even after much effort. And earler matterstates contained in memories required evolutions to even start being experimented with, they could be read to a certain extent but not really acted on. Even the reading the matterstate seems to be improved as well with 1st evolution it required we absorb Kennys brain to get some sort of understanding while lately a few drops of blood or other matter is enough. In 5th evolution perhaps a single molecule is enough. The question is how far does the rabbithole go with this understanding. Could you at some future powerlevel turn people or other matter into a previous matterstate? Not perhaps playing with the timeline like some superhumans but simply go "Yeah the matter that makes up your body just turned into when it was sludge at the bottom of the sea in the cambrian era". And combine it with the third having a "yeah Imma go turn into this neutron star 3 feet away from you. Have fun with that gravity".
Anyway thats my rambling speculation.
