Under my scenario, what would Ceredigion's network look like?
Virtually non-existent, the money's just not there to fund anything more than a line from Aberdyfi (Cardigan) to Aberystwyth and maybe a couple of small lines around the immediate area of Aberystwyth itself.
The entire idea of this system also completely implodes in a lot of areas- Take the Derby/Nottingham/Leicester area for example. The national lines might just stop at the three county towns (Rutland probably doesn't get anything under this system), but then you get the more regional centres. Lougborough will want links to each city, Ilkeston to both Derby and Nottingham, Long Eaton makes most of the Lace sold in Nottingham's Lace Market but is over the county boundaries, Chesterfield will want lines to both Mansfield and Sheffield, Doncaster with Worksop etc.
Lets assume for a moment that the national line out of Leicester takes a stop in Loughborough and then splits with one line going up to Nottingham and one to Derby. The Derby line then goes up to Sheffield via Chesterfield, then the Nottingham line heads to either Sheffield (via Mansfield and Chesterfield) or Doncaster and York (via Worksop), or possibly both splitting at Mansfield. Either way there's then a Derby-Nottingham line which we'll say goes through Long Eaton and Beeston.
Now, while you might have express trains doing just the major cities, there's no point in having a second line just doing all small settlements along the line between Derby and Nottingham (Derby-Chaddesden-Spondon-Borrowash-Draycott-Long Eaton-Attenborough-Beeston-Lenton-Nottingham), so they'll probably just run another train along there which (under this system) might be joint funded by Derbyshire and Nottinghamshire. Similarly the Nottingham-Loughborough-Leicester and Derby-Loughborough-Leicester lines will probably do the same thing. Then Ilkeston wants lines to both Derby and Nottingham, and the easiest thing to get to Derby is to just pull a spur out from the mainline at Long Eaton. Meanwhile Ilkeston is right on the border so lets assume that Nottingham County Rails have a terminal in Ilkeston, and Derby County Rails have this spur from Ilkeston down to Long Eaton. Now finances are tight (because it's being raised purely on the County basis), and so Derbyshire's only got the funding for one line at the moment, so lets assume that they put one in running from Long Eaton up through Ilkeston, Heanor and Alfreton towards Chesterfield. Two problems spring up though, the first is the problems of terrain in north Derbyshire- lots of tunnels and so forth, which means the easiest thing functionally is to have a junction with the mainline south of Chesterfield rather than try to get this secondary line up to Chesterfield station itself. Meanwhile Derbyshire is already running services on three branches of the National lines (running towards Sheffield, Nottingham and Loughborough) and running an entire new train and carriages for this one line is going to be expensive. All told, far cheaper to just use the same gauge as the national network.
Right so now we've got Nottinghamshire. We can assume that a line to Sheffield would be considered part of the national network, so that's on the main gauge, and if Doncaster isn't in the national network then it's still easiest to get to Worksop via Mansfield, and nobody wants to change trains within the county so that suddenly means that Worksop's on the main gauge as well.
And suddenly we've got a situation where Derbyshire and Nottinghamshire have both decided via precedent to use the national gauge. Now Leicestershire gains nothing by using her own gauge because that means her own businesses are losing out on access to the Nottingham Market compared to Derbyshire, so they'll want to use the same gauge as well.
And there's similar examples on multiple county boundaries across the country.