Or that, you know, maybe ray was put on Jakku to escape the first imperial recruitment system. Ray is on the opposite side of the coin to fin. The shuttle that dropped off ray doesn't look to be from imperial or Republic/Resistance origins which could suggest that ray was dumped on Jakku by her previous family.
Why is Ray force sensitive?
Anyone can harness the force. Hell Chewy and Han could be force users respectively. This is apparent in the Knights of the Old republic games, where you were able to completely turn your party into force sensitive users/Jedi/Sith if they weren't already. (Excluding droids of course, because they aren't considered "living"). Also, the force is naturally present around every living creature just like Mahz described. Some people are more force sensitive than others, for the longest time, the trailer had us thinking that Fin was supposed to be the lead role/Jedi type of deal.
Could Fin be a force user?
Probably. With training he could, if he went with Ray to find Luke, then perhaps he could've gone into training. I'm not directly implying he will become a force sensitive user, it's just a thought. Being traumatized can also bring out the force, light or dark, in some way. In the first scene of the movie, you could tell that Fin was willing to kill up until his buddy or former squad mate (unknown) was killed by a blaster bolt. I'm assuming it's one of his friends because he kind of reaches out to fin before he dies as a "help me" of some sorts. This is probably what "Woke fin up". He realized what he was doing was wrong and this forced him to truly open his eyes and realize what he's working for. We'll have to see.
Where the forget is the Rebellion?
It's probably evident enough that we don't see a single Resistance/Rebellion Flag "ship". Aside from the X-Wing squad lead by Po. But, where is the rebellion? Now, in the expanded universe, the Empire truly never went away. Palpatine had clones of his body and his "ghost" essentially possessed those husks whenever he was killed. The Empire split into multiple branches and segregated across the galaxy pretty much. Admiral Thrawn lead the First Remnant of the Empire against the rebellion, so we can only assume that the Rebellion is somewhere other than the current system Jakku is located in. Maybe the First Order is one of those segregated imperial regiments, i don't know. The only ship they posses though is roughly a modified Star Destroyer, probably a branch model.
There are so many things in this movie that i missed that i had to go back and watch it two more times. It's good for what it's worth.