You would still have to change trains though. The only route for Bangor-Cardiff that works without a change of train is the current one (via England). As well as Bangor-Porthmadog (please let's not have the detour via Afon Wen if this were to go ahead) and Aberystwyth-Carmarthen you would also need a new bridge across the Dyfi if you want to avoid passengers having to change at Dyfi Junction (I'm assuming the train from Cardiff would reverse at Aberystwyth, but the track layout at Dyfi Junction doesn't allow you to go towards Pwllheli from the Aberystwyth platform). The train would 'reverse at Aber', as it does when Aber train goes east from Shrewsbury. No problem with multiple units. If you have a new link within Wales, you would not have to change trains if TfW were to run a version of the Red Dragon.
I don't think there's enough space between the Dyfi bridge and the Cambrian Coast platform at Dyfi Junction to fit a chord in there. A new bridge there would almost certainly be cheaper than one near Borth across the mouth of the Dyfi though. The problem the original surveyors had with the Dyfi, is they couldn't find the bottom of the estuary! Also, the bridge was due to rejoin the Coast line close by the now built around Penhelig station area, whereas running a connection from the Aber end of Dyfi platform to meet the bridge would be cheaper maybe.
I think I disagree; although it depends on how you define 'large scale'. HS2/Crossrail levels of investment are without a doubt dubious for such areas, but I would very much hope for a massive uplift in bus services (which in comparison with other bus schemes could look like a 'large scale investment', despite being a fair bit cheaper than a high-capacity railway) with a massive improvement in the quality of vehicles (mainly better legroom) used on long-distance routes.
I agree, they should have started with Bangor-Caernarfon. I think the two should be the same project and I thought The Traws Link was promoting both. Sorry, but the southern boys have been better at PR I think
Road transport is devolved. However I believe that rail infrustructure through Network Rail Wales (unlike Network Rail Scotland) is not devolved and that the Welsh block grant (UK government funding to the Welsh Government) does not include an allocation for rail infrustructure. Remember that electrification to Swansea was cancelled by the UK government, not by the Welsh government. I'm not sure what the arangement is now that the core valley lines are transferring from Network Rail to Transport for Wales though; will there be an increase in the block grant to pay for core valley lines infrustructure?