During mc fight against Deryl ,just before is 3rd evolution .
a good thing to remember about that is what Sylla said about these memory
So it's not like he's going to have acces of all the power of people close to him at whim and i would expect at first mc able to use some basic capacity of each trait and train and finding a balance with all of them, in a similar way to what he did in the begining when he tried to copy other animals abilities but this time with other trait (like teleporting (but on little scale), absorbing energy like Michael, or the passive protection of Alice at first and the furter we move in the story, the stronger mc get and the better he will become with these new copied capacity , to a point when he will mastering them an be overpowered anyway at the end of the story.)
There is that A rank monster looking like Sylla that can copy any people from the memory of her target;but limited by his own pool of power.
Whether or not she is connected to Sylla, i would expect it to be closer to memory than mc and therefore capable of more things with memory trait.
So another possiblity is that Mc won't be able to do the same thing and he could be limited with some passive or basic capacity of each superhuman/monster from whom he possess memories ,which will still remain extremely usefull and adding versatily for mc without killing interest of other superhuman (because if mc can do everything Alice or Michael can do, with him beeing one evolution above them, showing their evolution as superhuman in the story will become almost meaningless )
The most problematic part is about it altering mc balance between is two trait, because with Sylla, we know that she lost her purity because of things she's done before meeting Stephen, and since she was chosen of the 4th (so she was pure memory) i think that it would be weird that using her trait would taint her purity, but since we dont know anything about that part of Sylla's past, we can only theorize about it .
But i would expect Sylla to warn us about it if they where any problem with that since she want to help us to be as close as possible to 50-50.
And for your question at the end, from what Sylla said mc should be 50%body since his father was a perfect match with it but Sylla isnt, she's more geared toward memory and purer than most, so we could expect something between 60-80 + something else, so mc should have inerited Sylla impurity too, and in theory he souldnt be able to be a perfect 50-50 either and it's seem like it's what Sylla implie.
Sylla mention that mc body as moved to forward, and now she try to counterbalance it with memories to "balance everything out" and then hopping that the 4th will be satisfied.
The question is, would it supress what remains of the other trait if mc would focus on one or would it work like some container filled with several component until he's full (full here mean reaching is fifth evolution since it's when other power manifesting stop) and only balancing with what's already there. The most logical answer would be the second and from what Sylla said about "balancing everything out", it's seems like it.
And the question about becoming pure could be answered with just that, but also with Sylla herself, since she somehow lost her purity, what could stop her to restore it if it was just about getting some good memories like she do it now with mc? in theory nothing and yet she didnt, but maybe she just died before having time to do so or found another purpose with Stephen and her son.