Ada is a BSAA agent now, Chris knows her and in no way he'd distrust her. Carla has identical looks to Ada, meaning tricking Chris into thinking it is Ada, she uses Chris' team to her advantage, which is to fight off those BOWs. In the end she traps them and kills them, escapes.
Though I really wanna know if she's going to be playable in the campaign. I'd happily pay my dollars if her campaign was DLC along the line.
18 months is either an error on IGN's behalf, or Capcom changed the dates. Because previously, it was 6 months (Serbian outbreak Decemeber 21-25th 2012, 24th Chris team is murdered, 25th they leave) and Lanshiang gets infected on June 30th 2013.