I consider 2 wires to be essential. The alternative doesn't fail often, but the failures are never at a convenient time.
I routed my cable (1) under the down tube, (2) along left chainstay & (3) along mudguard stay. Fixing methods were (1) patches of sticky tape, plus the odd cable tie, (2) 100% cover with strips of Sylglas tape which is initially transparent, but looks dirty after a few years. Mine has suffered mechanical damage (or failed without external assistance, which seems unlikely, given its intended usage), but the bell wire has survived. (3) spiral wrapping, intended AIUI to wrap bundles of wires. The smallest size from Maplin, works well along a mudguard stay. It's fiddly to install, but looks reasonably neat & offers a fair amount of protection from mechanical damage. It'd be a bit of a pain to redo if e.g. the stays had to be replaced.
I'm not convinced about the virtues of co-ax vs bell wire. The wiring is vulnerable to mechanical damage & assorted causes of high temperature
. IMHO bell wire is likely to be more tolerant of both, given that the worst case is failure of insulation between the conductors, which disables the front light IME.
Which rear light are you using? I'm convinced that mounting on the mudguard is by far the best option for visibility, but have been disappointed with the optics of what I've bought.