"I think Sophie pretended to be Alice so she could get her appendicitis fixed at the army hospital"
Two snags with this:
a) no way of planning that a girl in that condition would be found conscious (and indeed she wasn't) and an unknown casualty would be picked up by German paramedics (as she was) until identity established.
b) there are no British military hospitals in Germany any more (they were all closed down about 10 years ago) and all that left are medical reception stations (which are essentially little wards on back of medical centres used mainly for single soldiers with minor conditions who do not have anyone else to nurse them or whose accommodation is unsuitable during illness/convalescence)
And Baptiste of course went to the reception desk of a German hospital when he first attempted to visit 'Alice'
There's far too much to be covered in the last episode, unless it's going to go at a heck of a pace.
We need to know why and how Reid died, how that fitted the timing of the reappearance of 'Alice', what the original plan was, and how it changed when Sophie said her real name in the ambulance.
Also how far the brigadier was involved - the turtle story and the edge in his voice when he called a different girl 'Alice' indicate he must be, but we don't really know extent.
Why was Lena taken?
Why were the three of them trusted to be on the roller coaster? (though of course we have only Sophie's word about the circumstances of that visit). I did wonder if Lena was taken to be a replacement Alice if original Alice died some years ago, but that doesn't work as the three of them were in public together.
If he wanted more than two, why three? Or many?
Any other casualties? And will there be justice for the lovely policeman?