I completely understand the worry about safety as well as the idea of a precious year - the years vanish, not just the terms.
In terms of safety, all one can do is take comfort in the statistics, that the overwhelming majority of students survive their year out unharmed. These streets in Cowley really are student ghettos, which provides a safety all of its own. I like other areas more, especially Jericho and North Oxford, but to not live in Cowley carries the real risk of isolating a student whose college peers tend to all live that way.
It seems the best thing to do is go with it, find the best house you can, make the best of that house, and cross your fingers for coming high up the housing ballot for the third year - which should produce a room on the front quad and a shower!
And also to realise that many students do actually like the experience, even if their mothers can't quite see why!
Premier does seem a reasonable agent. Its student list comes out anytime now. If your DD seizes on the list and she and her friends go and view asap, they should do ok. You ask for large rooms, but there's always a mix, since the houses are old. Most have long thin gardens and all can be cleaned up and prettified - whether they then stay that way is up to the students!
I'm not sure there's a much better option. Mine are clear that they'd hate to have been away from everyone else.