I only know a little bit about (the arcane, ridiculous and unfit for purpose, IMO) RoW stuff. But (IMO, AFAIK etc):
1) OS mapping is not definitive. For that you need to consult the Definitive Map (digital or paper) that is held by the appropriate local authority (as others have mentioned). I believe you should also look at the Schedule that accompanies that map and further, though happy to be corrected on this, I believe the Schedule is "more" definitive than the Map.
2) Riding is not "illegal" if it is not recorded as a RoW category that includes for cycle use. It is just not permitted and, except where there are specific by laws and some other special circumstances, the offence is civil and therefore only liable for damage etc (probably telling granny how to such eggs on that one) all you're likely to get is an earful and to be asked to leave.
3) The designation of a RoW does not remove any higher rights that may exist. So for instance, where there is a footpath shown on an old drove road or similar then there can be legitimate access but it is not yet recorded.
As for the Ramblers antipathy, well, mleh. I sit on a LAF and there's as many pro-access for all as anti r/Ramblers reps, in my small sample size and personal anecdote experience. Funnily enough probably a similar distribution of clint-ish to responsible riders within the MTB group (and don't forget most of the group stuff is bollox because most people are walkers, paddlers, riders). Quite frankly I can't be arsed with building walls between user groups but hey, that's me
Adrian - you seem to be doing a good thing and to have considered many aspects

It also speaks volumes that you've engaged on here in such a balanced and positive way

If you are stuck or need assistance Kieran Foster at Cycling UK is very good at this stuff, has been instrumental in some long distance cycle routes and is also just a really good bloke. Though like all RoW nerds, they can really nerd-out given half the chance ;-). You can reach him through C-UK's contact pages. If you mention Tim from SingletrAction it might help, but then he's met me so it might not ;-)