With Aberystwyth going up to hourly all day, and also WMT doubling their services between Shrewsbury and Birmingham, that should provide plenty of relief for the Cambrian.
Crewe to Manchester makes up 30 miles of a 233 mile journey to Milford Haven. Newport to Swansea would add another 57 miles. That's not a lot of milage to justify the expense and weight of carrying bi mode equipment around to mostly not be used.
WMT doubling trains up to Shrewsbury might help up to Shrewsbury but not beyond shrewsbury up to aber and Pwllheli.
people who currently go to aber on the pwll/aber splitting service wont all suddenly decide to get the alternate aber only service between Shrewsbury and aber freeing up space on the pwll/aber splitter service.
any space freed up on the splitting service is likely to be filled fairly quickly with new customer gained from the hourly service and the new station opening at bow street.
to be fair the 20 less seats per train quoted by someone is quite significant, while there may be more standing room you surely cant create more capacity on line like the Cambrian line by making more people stand for long lengths of time.
having all the new civitys fitted with ERTMS tech would certainly reassure people that when needed extra capacity can be added April-September , giving people less seats on less available trains for that line isn't reassuring for its users!
sorry for rambling