I am also 100% sure that Ellie is the MC's daughter.
I am also almost certain that nothing ever happened between Olivia and Lucas and Olivia waited all those years for MC and was faithful to him.
After this update it also seems to me that even Ellie knows the truth about the MC being her real father, and the fact that she is officially sticking to version that Lucas is her father is probably only because that Olivia told her to do so.
So in my opinion only MC and Mia don't know the whole truth because MC doesn't know that Ellie is his daughter and Mia in turn doesn't know that MC is her father and also Ellie's father.
Another point is that it can be clearly deduced that Lucas has long had a crush on Olivia, probably from the very beginning when he and the MC met her.
And it seems to me that the whole plot in this game is based on the fact that here everyone is lying to everyone else and hiding secrets from others.