Tying together unrelated scenes is a pretty common practice in movie and game trailers. So long as the spliced in scene looks like it can match up with the scene before it in some way that makes their differences indiscernible, especially at a passing glance. In this case the walls, lighting, and tone are pretty similar. Considering the fact that the scene plays so quickly it's likely they were banking on people not noticing.
But just to be sure I decided to check for myself and played Jake/Sherry's demo to compare it to the Comic-con trailer. The scene at 1:02 was directly lifted from the Ustanak scenario. The Ustanak kills a man, causing him to scream. This prompts Jake to look back while Sherry, in her white jacket, can be seen in the background. This matches the scene from the Comic-con trailer exactly.
You can see the scene in question in this video walkthrough of the demo at about
1:12
So unfortunately the snake and Sherry scenes are taking place in two completely different areas. Of course that's not to say the snake won't appear elsewhere. If the large humanoid boss in Chris/Piers scenario appears when Sherry's present then it's possible that the snake monster will also appear again. Hopefully while a female character is in the party. I'm keeping my hopes up.
As for the female monster it could definitely be the woman with Helena.
Carla Radames could also be a candidate. Information seems to strongly hint at the possibility that Carla will appear in Ada's campaign. I imagine a Code Veronica type end in which, much like Alexia, she willingly infects herself. It'd at least explain why this female spider monster seems to accept what it's become, assuming the "breast caressing" is intentional. Either way there's at least a very good chance that whatever this boss is it'll appear in a scenario featuring a female character.