Highly unlikely theory:
Paula is recovering from a profound mental breakdown.
The confusing timelines and similar-looking characters (some imaginary?) reflect her fragmented, possibly at times psychotic, state of mind. And different elements of her own personality.
Her actual, RL Psychotherapist is Beth (Siobhan Redmond). Older woman, safe, supportive, mentor-figure. Paula's view of what a mother should be.
Beth is helping her with unresolved issues relating to trauma in childhood:
suicide of her own mother (as represented by Kay's death)?
Neglect (e.g. as represented by 'abandoned' baby Lia in car/ deprived of her mother's comfort whilst she's out for hours)?
Abuse (maybe in episode 3) ?, hence Paula's obsession with re-framing all mothers/ motherhood to a rose-tinted ideal.
And thus she is intolerant of anyone's transgressions or lack of enthusiasm for the role (Ellen. Who may represent Paula's conflicted career vs child dilemma, too?).
Was she unable to have a child of her own? Did she have one, but her early experiences repeated themselves and that child was removed? (e.g. as represented by Ellen's SS investigation). "The Replacement" may also refer to her fantasy of the (replacement) life she always longed for.
Aaarghhh. That still leaves so much unexplained, though.
I definitely believe David actually wanted children, despite convincing Kay he was content without any. His feelings around Lia have become broodier by the minute. Relevant, I think.
Very telling that Kay stated to Ellen (ep. 1) "...and I am speaking for David as well as myself...", in defence of her "smart choice" not to have kids.
Caris's replies to Ellen (on office visit) seemed very stilted/ rehearsed?