I give up

, I need to go and get a degree in ticketing, these rules are so random
What you quote are not rules as such. But yes railway ticketing is very complex. The best way to use this forum is to do a lot of reading. I'd start by thoroughly reading our Fares Guide (admittedly this is now out of date in some areas, for which I apologise. It is on our to-do list to get this updated).
Never assume you know the answer, and ask questions if you are unsure (where appropriate; sometimes a new thread would be required rather than going off-topic in an existing thread).
A few years ago I thought I knew a fair amount about railway ticketing, but actually at that time there was a lot I didn't know. The more you learn, the more you realise there is to learn.
Expect the unexpected. A forum member (not very active these days) had a saying along these lines: there are valid tickets that look invalid, there are invalid tickets that look valid, and everything in between.
The fastest route isn't always permitted, conditions are not always clear-cut, the shortest route isn't always obvious, negative easement (which are oxymorons) don't always have any actual effect, TOCs who price a particular flow may not operate any trains on which those fares are valid, some fares can never be sold by any online booking system as there are no services they are deemed to be valid on, and ... well, I could go on.
But I think there isn't anything else to be answered in this particular thread.