So, we have no actual figures to work with, so my questions still don't have proper answers.
Maybe that's because 'proper' answers to your questions would take considerable time and resources to provide.
It seems clear to me that any system, even one that's primarily based on mileage, would have to have lots of cases where individual fares are reviewed and adjusted manually to make sure they don't cause undue overcrowding, or anomalies where there are alternative routes. I don't think it's reasonable to expect people here to go through that analysis to come up with specific fares - which as I understand it is basically what your questions are asking.
We're individuals discussing, in principle, what a decent fares system might look like and on what principles it might be based. We're not train companies equipped with all the staff and detailed information (for example on line mileages) to work out the details of individual cases.
We will have mileage based pricing but at various different rates, we will have supplements for fast trains, and journeys like Poppleton to Sheffield have to be bought with a routeing of "Via: Harrogate/Leeds/Doncaster" or "Via: York/Pontefract/ NOT Leeds/Doncaster" or "Via: Leeds/Wakefield NOT Harrogate/Doncaster" and so on and so forth?
I'm not sure that's entirely fair to put it like that. Those suggestions have been made by various different people who have different opinions, and I don't think any one single person here has said they support
all of those ideas.
And I thought the
current system was complicated
What you've described above may well be complicated
to the TOC to work out the possible fares in the first place, but that's not an issue. The issue is how complex is it
to the passenger to work out which ticket is the best/cheapest one for him to buy. In that regard, I'm not convinced what is being proposed is more complex than what exists at present - where you don't even know at the ticket machine whether you might be able to get a better deal by split ticketing. (Although I would say that I think having very fine-grained choices of which route your ticket is valid on is too complex, I personally probably wouldn't support that).
Besides, I don't think it's just complexity. The question of
fairness is also important. No system of fares is going to be perfect - because of the need to balance competing requirements (demand management, ease of enforcement, complexity, etc.). But one thing that seems to have been completely lost from the current system is fairness. The example that sparked this thread was that on some Virgin trains from London, the walk-on fare actually gets
lower if you stay on the train for longer. That may well have good rationale in terms of yield management etc., but it runs completely against the human instinct of fairness. It makes the people who are paying more to travel the shorter distance feel (arguably, justifiably) ripped off, and leads to all sorts of attempts to work around the system, and can only damage confidence in the railways. Split-ticketing is another example: The fact that it is possible runs against basic notions of fairness - in some ways that's a more significant problem than the fact that split-ticketing can be so complex to do.
That's all important. And it's something that a system based (roughly) on mileage, would be likely to fix (although obviously, care would be needed in working out the details).