We don't know what reasons one sister gave for wanting leave. I would imagine she wouldn't need to state a reason, but may have. Quitting would draw a lot more attention to her. Colleagues would ask where her new job is, maybe throw a party or have dinner/drinks for her. None of which would likely be wanted if she wanted to quietly leave IMHO.
Eliza left a phone behind in the flat. I'm not certain what other electronics were left, but I thought I read that there was another phone as well. I'd guess there's probably a laptop or desktop computer too, or do people rely just on phones these days?
I think it's been discussed quite a lot (by locals here as well) that it would be pretty difficult, if not impossible, to exit the area unseen. Police also have stated there's no evidence to suggest they left the area.
The day before, they apparently walked a fairly direct route back home, but through a city shopping centre. There's been no mention AFAIK of the last footage seen during daylight hours, but police have stated that they went from the bridge, through the shopping centre to home, and they don't believe the sisters left again until the early hours of the next morning where they were then seen back at the bridge. Police have a pretty good idea of their last known movements between roughly 2pm and 2am.