The episode finale takes place 62 years in future, yet the house looks just like it did in the present day, with no future technology or anything. That's because it's a dream.
Fridge Brilliance and Fridge Horror: Father Christmas first showed up at the end of the previous episode. Santa represents the dreamer's mind trying to fight back...which means the Doctor was in the dream then. Which means the crabs got him before the end of the last episode.
In the realm of her own dreams, there really is nothing more important than Clara's egomania: why else would dream-Danny insist that he'd died to save her and her alone, even though the real Danny always put his responsibility to his students and to the boy he'd killed in Afghanistan above his and Clara's personal needs and wishes? It's only when dream-Danny becomes more defined and realistic that her wish that he'd put her above everything else gives way to her respect for the real Danny, who would want her to move on and find happiness without him.
Of course Clara wasn't fully awakened by riding in Santa's sleigh. "Santa", to her, looks like the Doctor, so she needed to wake up one more time by having him rescue her for it to count.
The Doctor's remark about Alien - "That's really offensive, no wonder everyone keeps invading you." - may be close to the truth. The film was released in 1979, which, depending on your view of the Unit Dating controversy, lines up well with the first of many alien invasions the Doctor and UNIT had to deal with together.
If you take the fruit at the end as a sign that Santa was/is real, Santa was apparently in Clara's bedroom the same night the crab attacked her. Did Santa Claus deliberately get Clara and the Doctor trapped in a shared dream-crab dream as some sort of crazy plan to get them back together? Did he do it so Clara could save the Doctor and the Doctor could save everyone else? Either way, what the hell, Santa?
From the fact that the other victims were not shown after the penultimate dream layer, it could be implied that the crabs created the final layer of the dreams on the fly, and it was a very nicely made trap tailored to the Doctor's fears and weariness. How do you win when the enemy can break the rules so effectively? Can you ever be sure that this is it, that you're awake now?
, lines up well with the first of many alien invasions the Doctor and UNIT had to deal with together.