Your questions answered - NOTE SPOILERS AHEAD
Why did Han Solo have to die?
It was a necessary part of the plot. For Kylo Ren to complete his journey to the dark side of the force he had to kill his father. This was reminiscent of scenes in Episode 6 when the Emporer encourages Luke to kill Vader.
Who are Rey's parents?
There is speculation that Rey may be Luke's daughter. However, I believe Rey is Leia's daughter. This makes more sense in the wider context of Skywalker family conflict. The fight between a brother (Ren) and a sister (Rey) is more powerful than one between cousins.
The relationship between Leia and Han became fractured when their son turned to the dark side. But there could have been moments of reconciliation during which time Leia conceived Rey. Therefore Han is Rey's father. I like this theory more than the one where Leia has an immaculate conception similar to Anakin's mother Shmi Skywalker.
Leia chose to hide her pregnancy from Han in order to protect her daughter. Han's mind could be read by any dark side force wielder and this information could be used to turn Rey to the dark side. Leia gave Rey some basic Force training as a child which is what 'awakens' during the film. Luke probably gave Leia some basic training after the Battle of Endor. Leia chose not to complete the Jedi training in favour of being a major part of the rebellion. She couldn't do both.
BB-8 was probably around when Rey was growing up. He belongs to the rebellion and Rey could miraculously understand the little robot.
As the First Order and Kylo Ren grew in power, Leia became increasingly worried for Rey's safety. She sent Leia to live with a trusted friend. I make a leap here to connect the dots- bear with me- she sent her to live with Llando Calrissian somewhere in the Western reaches. Llando had a son and Rey and Llando's son were playmates and friends.
We know the First Order's recruitment methods were similar to those in 1939 Germany, forcible separation of very young children from parents and brainwashing of these children into the Order and their inevitable grooming into docile stormtroopers. Around the time Rey was five, the Fisrt Order came a-recruiting. They take Llando's son but Llando's manages to flee with Rey and deposits her on Jakku promising to return for her. He probably dies and she has to fend for herself as a scavenger. We see scenes of her being deposited in haste in Rey's flashback images.
Who is Finn?
You've probably figured out by now that he's Llando's son forcibly removed from his parents and brainwashed into the First Order. But why is he the only Stormtrooper who has a conscience and rejects the Orders methods? It is because he is Llando's son, he would have been brought up with tales of the brave Jedi and the struggle against the Empire, including his father's role. He can't reconcile this with the Order's doctrine and is conflicted and decides to rebel.
It is no coincidence that Rey and Finn are brought together. They were connected as children and as Rey is Force sensitive she draws him to her without knowing. There is a love story blossoming between these two which would wrap up Episode 9 in a neat Disney bow- Leia and Han's daughter marries Llando's son... sigh.
What can we expect in Episode 8?
A reluctant Uncle Luke will train Rey in the ways of the force. The mysterious Supreme Leader Snoke will complete Kylo Ren's training. The First Order will build a new Starkiller base and leave the Galaxy and the last vestiges of the Republic in tatters, reminiscent of the end of Empire strikes back. Episode 8 will probably be called Star Wars:The Fury of the First Order or something to that effect.