NOOb here. Just thought I would give my 0.02.
I think the symbiote will come from the moon in the shuttle that crashes. And in the crash, it gets loose and someone finds Peter. In the first trailer Peter is laying in bed in his Spidey suit and you can see the symbiote coming off the floor and attaching to his arm. I think this will be point where he gets the symbiote.
Now, whatever happens in the movie happens but I think that we will not see that much of venom. In the Comic Con trailer, you hear Peter saying that he has to stop it, but I don't think he is talking about Venom, I think he is talking about himself. After defeating Sandman and almost killing (or killing) Harry he come to realize that this symbiote is not something he wants to be a part of and tries to get rid of it. The scream you hear at the end of the 2nd trailer is him getting rid of the symbiote.
Now when we see Brock in the church asking for someone to kill Peter, I don't think he has yet to bond with the symbiote. Maybe in the movie Peter does something horrible to Brock while under the influence of the symbiote and Brock wants Peter to die because of it. Just by coincidence, Brock is in the same church that Peter is in when he gets rid of the symbiote. In the first trailer you see Brock look up and say,"Peter". I think he is seeing Peter tear off the Black suit and realizes that he is Spiderman. Now the symbiote falls after Peter tears it off and just by luck lands on Brock. It takes over Brock and because of Brocks pure hatred of Peter, turns into something much worse than Black Spidey, Venom.
Peter thinks everything is ok but then realizes that there is somethign out there much worse. I doubt Venom will have that much air time and the last battle royal will end with the symbiote being "unbonded" with Brock and possibly someone dying???
Or maybe I need to stop smoking crack.