I'm still really confused about so much of this. And I think we're meant to be.
So, Sophie was abducted from near Paris in 2002. A yellow camper van is associated with her disappearance. Sophie is 10/11 years old and has a French father and an English mother. So would (presumably) have spoken both languages fluently.
Not long after she disappears Sophie's mother commits suicide. Sophie's father clearly blames Baptiste (the investigating officer) for her state of mind, but we don't know why.
In 2003, Alice, an 11 year old English girl living in Germany with a history of conflict with her family and playing truant from school is abducted / disappears. A yellow camper van is associated with her disappearance but this is not known to the police. Alice and Sophie are pretty much the same age and so alike that they could be sisters. Oddly the two abductions are not linked.
One of the girls turns up in 2014 claiming to be Alice and to have been kept with Sophie. No DNA test is performed as Alice's family say that they recognise her.
I think that the evidence (finger tapping, mother's suspicions) is that the returned girl is Sophie not Alice. For some reason therefore Sophie must not have wanted to return to her own family or to see them. Maybe some kind of abuse or were her family involved in her disappearance. Could that be a motive for her mother's suicide? Perhaps Sophie's mother found something out that led her to assume that Sophie must be dead and that she and / or her father was responsible.
Sophie can only take over Alice's life if Alice remains hidden. Did Sophie do some kind of deal with her captors? Sophie only seems to have led the police to the hiding place many days later. Was she giving her captors time to escape, get rid of evidence and move Alice?
When the returned girl who pretends to be Alice talks about Sophie she says that she was confrontational and argumentative with the captors. This behaviour seems more like the 11 year old Alice that we saw than the nervous fidgeting Sophie on the video we saw.
The hospital say that the returned girl has given birth, yet she denies it to Alice's mother. Is this because true Alice has NOT given birth, Sophie has. In pretending to be Alice, Sophie has completely taken on Alice's story so denies the birth.
Sophie dies (somehow) on 22/12/14 still playing the role of Alice, so is buried as Alice. Only Baptiste and Alice's mum believe that Sophie lies in the grave. This means that Alice is still, potentially, alive.
Henry Reed's father is said to have committed suicide but Henry doesn't believe this to be true. Was Henry's father killed? Who by? Was it because he knew too much.
Could the Roger Allam figure be pretending to have Altzheimer's so that he doesn't get bumped off too? He clearly has warned / threatened the returned girl to keep her mouth shut.