An interesting idea, but Newcastle Emlyn was a branch off the Aberystwyth-Carmarthen line, not part of the through route. If you wanted a phased re-opening, I guess the first stage would have to be one of Carmarthen-Pencader, Carmarthen-Lampeter, Aberystwyth-Llanfarian or Aberystwyth-Llanilar. All of these suffer from including one of the 'expensive bits' where a new route would be required. The old route has been built on at the Aberystwyth end, and at the Carmarthen end (south of Pencader) I expect the curvature of the former route would significantly lengthen journey times. If the train isn't faster than driving there is absolutely zero case for re-opening.
I'd start by upgrading Cardiff-Bridgend, and Carmarthen-Port Talbot (via the Swansea District Line), with a target Carmarthen-Cardiff journey time of no more than 70 minutes (I'd call it 'Project 70'). Then keep the waiting rooms at Aberystwyth and Carmarthen stations open from first to last bus/train, and put on faster (more-direct) buses between the two with enhanced legroom etc, connecting with the fast Cardiff-Carmarthen services. Then Bangor-Caernarfon with an hourly service initially but sufficient capacity for at least 2tph, with passive provison for extension further south to Porthmadog (using a new route between Bryncir and Porthmadog, rather than reopening via Afon Wen which would be slower, again there's no point building a railway if the car is faster, you might as well invest in buses instead) and/or Llanberris.
Why do you love Wales? If you build bigger roads, more cars will come to spoil Wales, and the climate.
You don't have to do that, the rubbish toilet-less bus from Aberystwyth doesn't go all the way to Cardiff, it terminates at Carmarthen and you have to change. So change onto a train (which will hopefully have a toilet and, if you're lucky, won't be a rubbish train (there are some rubbish trains on the Carmarthen line though)) at Carmarthen instead of going all the way on a bus.
Carmarthen and Aberystwyth are not 'two remote small market towns, socially and economically remote from each other'. In the context of our area, they are large towns, and the hourly bus service between them was the busiest TrawsCambria bus route.
Interesting question: how many cars does a class 158 need to take off the road to result in reduced greenhouse gas emmisions overall? Obviously, in rural areas the best solution would be to get pepole onto buses instead of driving, but buses are always slower than driving some that might not be achievable. Trains are much more likely to achieve modal shift.
There's no through demand on the Scotish border's railway is there? Maybe there would be if it ran all the way from Carlisle to Edinburgh, but it doesn't do that currently, yet sounds like it has been a huge success. The borders to Edinburgh sounds compareable to Aberystwyth and Lampeter to Cardiff to me, but I suppose Cardiff is not Edinburgh and Aberystwyth has alterative major centres (Shrewsbury and Birmingham) for passengers to head for, which may split the demand. Still, the Aberystwyth to Shrewsbury and Birmingham trains are doing well...
I don't know about the Dolgellau improvements, but the others all appear to be further destruction of the competivitiveness of public transport. Bypasses, I believe, make things very difficult for buses. A Bangor-Porthmadog railway would probably be more successful without Caernarfon and Porthmadog bypasses too, and the dual carriageway across Ynys Mon probably impacts the railway to Holyhead as well.