This is an interesting theory. Despite being 32, they could be seen as possibly vulnerable. No relatives in the country, maybe minimal friends? Kept mostly to themselves perhaps. We don't know how well they spoke and understood English either. We also don't know their intelligence level. That's not a dig at them, it's just a fact that not everyone is highly intelligent and we know nothing of their intellectual abilities. Maybe they just didn't have any kind of "street smarts" or were too trusting? Perhaps they had no intention of leaving their flat by the Tuesday (they certainly made no reference to it in the phone call to their mother on the Saturday prior) and a mysterious letter was written and sent by others claiming to be them. Maybe the main point of contact between the women and the land lady was usually via the phone left in the flat, so therefore no response? However, this wouldn't make sense if a drug gang wanted to take over the flat, they certainly wouldn't want anyone to think anything was amiss.
The fact that it was low tide around the time of their disappearance might hide any evidence of shoe or skid marks, or signs of trying to get hold of the edge once the waters rose. My thoughts go to what happens next, if they did indeed end up in the water. Regardless of accidental or on purpose, they would then float down the river, under the bridge we've seen in lots of still pics and out to sea I imagine? At what point would the cold water & freezing air take the life from you? Wouldn't your body naturally fight it, if it was intentional on their part? I have no idea on the mechanics of death by drowning, (accidental or otherwise) but I'm wondering how soon before they are fully submerged? What about lose items like scarves or the bag? Would it be possible to see anything floating down the river in any CCTV footage around that time? Is the harbour a busy area, with things happening after hours, or strictly in business hours/light?
As for what the woman are wearing, my thoughts are that their hair/heads look very similar - maybe a bun or tied up? It doesn't look like they're wearing hats to me. Each has a scarf wrapped around their neck, against the cold. Both have appropriate coats, one longer than the other. The trench coat appears to be belted around the waist. Both appear to be wearing long, dark pants, jeans maybe? The one closest to the building looks to have flat soled ankle boots on, the other is harder to tell. I don't see heels though, maybe something like a patent leather brogue, with the tiniest flash of white socks visible? (Or maybe just shine from lights) The one in the trench coat appears to have her left hand in a pocket, her right is hard to tell. I don't think she's holding anything, I feel it's just that she's retracted her hand against the cold, and it's the lining of her coat, captured at just the right time/angle to see right up into it. The other sister seems to have her right hand in a pocket, and her left doesn't seem visible to me. I would guess it's swinging behind her. The cross body bag she's wearing appears dark, possibly black. Not huge, enough to hold a phone, purse, keys and other bits and pieces us women tend to carry around with them.