Now forgive me if this has been suggested up-thread, I haven't the will-power to trawl through it - we're at 5 pages!
I came to Oxford in the mid sixties and at that time I think there were the remains of a level crossing immediately south of Botley Road bridge. I went to look at it at the time - there was a barrow crossing for sure, but then beyond that I remember the remnants of a much wider road crossing. I discussed it with a fellow student and he thought the wider crossing was sometimes used rather than the under-bridge for awkward loads and that sort of thing.
Now it's a long time ago, and I might be wrong of course. So I looked at it on Google Earth and indeed, it looks as if a temporary at-grade crossing of the rail tracks (there's four of them) and the approach ramps could be engineered. It looks as if there is space on both sides of the railway for temporary ramps. Such an arrangement would enable normal Botley Road traffic to continue, and avoid the need for long detours and the increase in traffic on other roads, and bus services could continue as normal. Now I know the railway wouldn't like it, they have to have rail access to the bridge site and would prefer to have it from both north and south directions. Nevertheless, it looks as if my at-grade crossing idea could be implemented and it would solve a bundle of awkward problems.
What do you think?