I hesitate to post anything on this thread because of a) the fact that - as others have said - friends and even parents might be reading it, and b) if one is not very careful in these situations it's potentially possible to compromise a trial or at least damage the chances of conviction by posting something that a clever dick lawyer can use to stand up in court and say "how can my client possibly have a fair trial in these circumstances, etc etc"
So I am really just posting on this to get my own ducks in a row and I will try and refrain from lurid speculation. Also as a native of Hull, I might be able to help with a little local knowledge. It goes without saying that I feel terribly for her parents and friends, it must be an unimaginable situation to be in, and I hope that, eventually, she gets justice, if it does turn out that way.
I've read through most of this thread and one thing that's leapt out at me is this business of a missing person's body being carried down Barmston Drain, into the River Hull, and thence into the Humber. While I am never one to say "never", I find myself on the side of those wondering about this. The land on which Hull is built, and indeed the whole plain of Holderness, is crisscrossed with shallow drains and drainage ditches. It's very similar in that respect to the area round The Wash.
I noted quite early on in the investigation that the police had frogmen searching Barmston Drain just in case and they found nothing. I haven't actually seen the dreaded sluice which has been mentioned so often of late but I imagine its main purpose is to stop old bike tyres and supermarket trolleys progressing further. I'd be surprised, to be honest, if any of the drains (by the way these are open ditches and not what you would think of as underground concrete drains, for those not familiar with this usage of the word) are deep enough or have a strong enough current to move anything substantial. I remember when I was nobbut a kid going to catch tadpoles in similar watercourses.
The River Hull is tidal, all the way up to Beverley Beck, if I remember rightly. But it is a river much clogged up with silt and mud. At low tide the water is just a fairly ineffectual trickle. Dropping something like a body into the River Hull is still not beyond the bounds of possibility, at least at high tide (what time was high tide that night?) but, as others have said, there is almost certainly CCTV on all the bridges - it might be feasible to access the river side via some of the old warehouses and streets round Wincolmlee without showing up, although there are businesses there that probably also have their own CCTV.
It is possible, as I said, to go into the River Hull at high tide. I almost did it myself one night when I was a young lad and we'd been out drinking in the old town area of Hull, and decided to go for a pee off Scale Lane Staithe. Fortunately, I kept my balance. But had it been low tide all that would have happened anyway would have been a fall of 15 feet or so into mud. Still not good, but as long as someone saw me and hoicked me out before the next high tide...
The River Humber is a very large and complex river, difficult to explain if you don't know it. It is a mile or so wide. Some people compare it to the Mersey, but the Humber is full of sandbanks and shifting channels that are always on the move, and the coasters and barges that ply up to Goole and back have to stick to the deep water channels or risk grounding. In fact, it is said that it was so shallow at some times that the Roman legions were able to ford it, marching on their way from Lincoln to York via the fort at Brough on Humber (Petuaria). Back in (I think) the 1940s, a bloke tried to emulate this to prove it had been possible, and he made it, although he was wading up to his neck in water at some points.
Given the shifting tides and sandbanks of the Humber, the main problem anyone would have in trying to get rid of a body straight into the river from the Hull side would be the ability to access deep water near enough to the shore. It's not so much that these places don't exist, there are docks, a ferry terminal, the pier near The Deep, etc etc. But CCTV and security exist at all these places. St Andrew's Quay, being the old fishing dock, might be a possibility because there is a lot of disused land down there. Not living in Hull, these days, I don't know whether there's security or not.
The most obvious thing to do would be to drive to the middle of the Humber Bridge over the deep water channel, but I would imagine that, if they haven't already done so, the police are even now trawling through the CCTV on the bridge cameras looking for anything odd.
I'm going to split this post here because I don't want to risk losing the whole thing when I press submit.