I have been considering about this and i have distorted the actual game's story so it makes sense. This is what i think that happened in the mansion (lots of things are different from what's seen in the game):
Jill and Barry enter the mansion. they hear the gunshots and enter the dining room.
Meanwhile, Chris and Wesker enter the mansion. Wesker orders Chris to investigate while he secures the Hall. Chris enters the art room (the one with the statue, which connects the Hall to the Cerberus hallway)
Barry and Jill find Kenneth's corpse. Barry decides to go and secure the Hall while Jill keeps exploring the mansion.
Barry meets Wesker at the Hall and that's when Wesker threats Barry. Barry agrees to help him betraying the STARS and they split up.
Eventually, Chris ends up in the infirmary, where he finds Rebecca. She tells him that Richard got bitten by Yawn while trying to save her (as in REUC) and she is looking for a serum.
Jill finds Richard badly wounded in the attic. He says he dropped the key to the graveyard (where he came from) in the next room, but yawn is there. Jill procceeds despite the warning. Jill defeats Yawn and got the key but, as she is leaving, Yawn rises again (she doesn't see it), and Richard enters just in time to sacrifice himself for her. Jill eventually defeats Yawn again and takes the still-breathing Richard to the previous room. He soonly dies and Jill decides to escape the mansion, in horror.
Soon after, Chris and Rebecca enter the room where Richard's corpse is, with the serum. Upon realising he is dead, Yawn breaks the door and attacks them. However, it's too weak and thay can easily kill it. As they leave the room, Rebecca tells Chris about Umbrella and the viruses and tells Chris she suspects there's a researching facility nearby. Chrisdecides too look for the facility to get some evidence.
Jill finds Barry, who convinces her not to go, and keep looking for survivors. They decide to explore the Residence (Jill has the Backyard key now).
Chris and Rebecca head to the cabin, where they find the Key to the Altar. In their way out, Lisa Trevor hits Chris, who becomes unconscious. Rebecca hides in the trees outside and, when Lisa is out of sight, she goes back to give assistance to Chris. Barry and Jill explore the caves. After shee gets through a dark place, she notices Barry has misteriously disappeared. She keeps exploring and witnesses Enrico's death (she thinks Barry killed him).
Chris wakes up to find Rebecca worried about him. They make their way into the labs, through the altar (they don't find Lisa there).
Jill goes back and hears noises coming from the Altar, and decides to go down there. Barry points her his weapon (as in the cutscene), and Jill gets angry, but they join forces when they see Lisa. They push her into the endless pit and Wesker meets up with them. Both him and Barry point their guns to Jill. Wesker orders Barry to take Jill into the holding cell, which he does.
Wesker manages to get to the research lab and inject the experimental virus on himself while Chris and Rebecca are collecting evidence. They find Wesker's name in some documents and realise he is involved with Umbrella.
Chris and Rebecca find Wesker at the research lab. Wesker tells them about his involvement with Umbrella as long as having lured the STARS to gather combat data. Rebecca asks for Enrico (she saw him alive in RE0) and Wesker says he shot him. He releases the T-002, which "kills" him and pursues Chris and Rebecca. They fight him for a while and Rebecca injects it with a drug she finds in the lab. The tyrants looks dead. Chris sees Jill through a camera and decides to rescue her and tells Rebecca to activate the self destruct system, planning to meet at the Heliport.
Rebecca activates the system in the machinery room (where there's lots of chimeras).
Chris finds Jill, and they hear the emergency warning and hurry to the heliport.
When Rebecca is finally ready to go, Barry enters and points his gun at her and tells her to follow him (he was going to lead her to the cells). She shows him Wesker's corpse through a monitor, and Barry lowers his weapon and apologizes. They head to the heliport. As they move away from there, The monitor shows Wesker rising from the ground (they don't see it).
Chris and Jill find and use the signal rockets.
The monitor in the machinery room now shows no sign of Wesker and suddently, the tyrant also rises from the ground.
As Chris and Jill start hearing Brad's helicopter, the tyrant brusts from the ground. Brad throws the Rocket Launcher at them, and they fail 3 of the 4 rockets, because they can't immobilize the tyrant.
Barry and Rebecca finally arrive from the elevator and Barry immobilizes the tyrant with his magnum. Chris hits the tyrant with the last rocket, killing it . Brad finally fells safe to lower the chopper and allow the fellow STARS to enter it.
Wesker shoots the chandelier in the main Hall, immobilizing Lisa Trevor. As he runs into the forest, he looks up and sees a chopper flying nearby, and realises Chris and probably some few other STARS members have escaped. He hears Lisas's moans followed by a big explosion, but never looking back or slowing his pace.
In the chopper, Barry apologizes his comrades and explains himself. They are angry at him, but understand he was in a hard situation.
Chris stares at a STARS picture and at a picture of Claire. Rebecca heals some wounds Jill has on her arms.
