I live in Bristol and have been following this case closely, hoping they find answers for Jack's family.
To me, the odd features in this case are:
1. Why Jack was wandering around the bypass road network in a direction which makes no sense; and
2. His phone activity.
It has never made sense to me that Jack would leave the party to try to get a taxi. Hailing a taxi on the road is very rare in Bristol - everyone orders Ubers. I would think it would be almost impossible to hail a taxi from the roadside on Hotwells Road, and absolutely impossible on the bypass roads across the river where he was wandering. The bypass road is a 40mph road with no stopping areas.
Ordering an Uber is far easier and more successful from a fixed location, eg the party. It doesn't make sense to leave the location of the party (in the warm) to try to get an Uber to pick you up from the roadside.
The only evidence we have that he was going to get a taxi home is his text/WhatsApp to his mum at 1.52am. But then he doesn't actually leave until c3am. Why? Probably because he wanted to reassure his mum, and he was having a good time. We know in the intervening period he falls down some stairs, bangs his head and shoves someone he doesn't know who mocks him. To me, these are the actions of a young man who is drunk and/or high - no criticism of him here, he's a student and it is to be expected at parties. I don't necessarily trust the word of other party goers who were also drunk/high that he was lucid.
So then when he leaves the party, what are his intentions? To get a taxi home, or to go to his brother's house a short walk away in Clifton? If he was going to get a taxi, surely he'd have ordered an Uber on his phone, but if he was going to his brother's, wouldn't he have messaged him to check he was in as he presumably didn't have a key?
This takes me onto my theory (which is MOO) that he may have lost his phone at the party between 1.52am and 3am, and left the party without his phone. The phone remains at the party or is taken by somebody who had been at the party with him. When his friend messages him, someone sees his phone light up. When she calls, they then answer and say "Hello". The reason there is no road noise is because the phone isn't with Jack on the bypass, it's inside someone's house. Maybe someone then has an attack of conscience and suggests calling Jack's In Case of Emergency number (his mum) to reunite him. With no response, they watch a video on his phone until the battery runs out. The next morning / afternoon with a hangover / come down, they hear that Jack is missing and realise it doesn't look great that they have his phone, and destroy it.
I know this is speculation, but bearing in mind the facts we do know, it seems to me to be more logical that a drunk/high student left his phone at a party than the other options.
The question is then, of course, what happened to poor Jack.
I do not believe that he was trying to hail a cab. If he was, he'd have headed East along Hotwells road towards the Harbour side / Corn St, where he may have had a chance. He would not have headed straight for the bypass. His route does start off looking like he intended to walk back to Flax Bourton, which is a c2 hour walk, at 3am, in icy/snowy conditions. Perhaps he got as far as Brunel Lock road, realised that was not going to work, and headed back on himself over Brunel Way towards his brother's house?
If so, the last CCTV we have of him is walking in broadly the right direction (albeit a round about way, perhaps due to the lack of Google maps) straight towards the lock over the water. There are no barriers around this lock, and in my experience the path can be slippy as it doesn't get much footfall in bad weather. It unfortunately seems most likely to me that he fell in the water. This seems more likely to me than him coming to harm at the hands of someone else, in an area with heavy traffic and as a young fit man.
