...now have a noise similar to a chain rubbing on a front mech when not set up properly.
Hi Stu! Welcome to the Forum!
I'm surely sorry you're having to deal with such a frustrating problem; I don't think there's much more difficult than trying to track down a Phantom Noise.
Reading your description carefully, I believe I'd center my next investigation on what is happening to the chain under pedaling pressure. By any chance -- and this is a long shot, I realize -- could the lever on the hub quick-release be on the right side? If so, there is a possibility the chain may be rubbing on it while under tension, accounting for the noise you describe. Also, if the q/r is set too tight, it can inhibit the free spinning of the Rohloff hub.
I suspicion your problem is something in actual contact. You said, "It is present all the time but obviously louder when under more pressure". It would be nice if that was the case, anyway, as it might be easier to locate and address than something internal to the hub.
You've changed the chain when you reversed the chainring and cogs, so that shouldn't be the problem. If you had put the worn chain back on with the reversed tooth faces, I might have suspected that.
Less likely, but another thing to check is the eccentric bottom bracket. It is possible one or both of the grub screws that secure the eccentric within the shell are loose, allowing the insert to rock under tension and produce noise. I realize you have checked it very carefully, but if the insert has been adjusted a number of times, it is possible the holes have run together and it is no longer being held as securely as needed under pedaling load.
I hope something in the above will help. If it doesn't, you can count on our Forum members to weigh in with useful suggestions and experience to apply. Please keep up updated as your efforts progress.
Best,
Dan.