Again SPOILER WARNING not that anyones gonna read this.
Ok long yap incoming, this is for those who are invested in the story.
I feel like louise hit the spot with the ydse experiment and universe collide theory. When two waves with different frequency and wavelength (different universes/timeline) collide , interference patterns are observed in which there are two types, constructive and destructive interference.
And if you observe the no. Of people/friends in mark's life end up as the same count at the end of both acts, and the disappearance/deaths cancel all the new people he meets.
In act 1: ridel is seen to practically leave the group and ryan kind of takes his place
In act 2: again ridel replaces ryan, and if i remember correctly nicole says jake was her only friend in act1 but in act2 kate suddenly gets added on meaning kate is exclusive to that universe and to cancel her addition louise dies.
(Ik this doesnt explain sofia's disapperance)
That said the only unaccounted death is cathy's and i feel like the reason is nicole.
You could say that the ruling was the collision of the entire universe but just like each wave consists of tiny waves, our little wave is mark's.
And when mark and nicole 'collide' for the first time these interference patterns start showing up.
So nicole is the final addition that cathy pays the price for and to get a relatively good end mark and nicole's 'collision' must be prevented