Pics there now since you edited the post - thanks.
It's looking great, and to be honest, once the rear glazing is in place, with the central, upper brace, the single cable won't be that noticeable, especially if the canopy is posed open. If you think you can modify it without causing damage, fine, but I think I'd be tempted to leave it as is.
One of my Spit models has a single cable, as one detached when doing the final positioning of the harness, with the fuselage halves already joined and, even though I know it's a single cable, it's extremely difficult to tell.
One thing I've just noticed though, although it's a bit late to rectify now - the 'Y' straps which are shown on the seat back -pad, should run behind the seat, and anchor on the lower cross-member. I do understand that the 'works' drawing is not that clear in this respect - larger, more clearly printed versions do show this, but need to be studied very closely !
Again, it's a tricky one to correct, but it might be possible to either remove them, or loosen the lower attachment, and push them behind the seat, possibly cutting off the 'hidden' portions if desired. They were virtually hidden from direct view, as they ran off the shoulder straps, straight over the seat back, and were mostly 'masked' by the drape of the shoulder straps.
For future reference, certainly for the Mk1 Spitfire, and probably for at least earlier MkII Spits - the oxygen hose was similar to that attached to the earlier oxygen mask of the period, in as much it was a 'plain', woven fabric, rubber- coated 'tube' type of hose, as opposed to the later-style 'corrugated' flexible hose, used on both the mask and the cockpit hose attachment. It was mainly black in colour, with a yellow and white 'weave' presenting a 'broken-striped' appearance, and around half the diameter of the later rubber and/or fabric covered, rubberized hose. (Again, I have pics, but can't get at them on my current Internet set-up !).
However, it's entirely possible that later-production MkIIs had the cockpit hose as you've modeled it, so it's not worth the hassle of trying to change it, especially after all the work involved in creating it.
Incidentally, Model Design Construction supply 'corrugated' hoses in a flexible vinyl type material, in various 'gauges' which can be used across the popular scales, and they're rather nice, and very useful - there should be pics on their web-site, as, yet again, I can't access mine at the moment.