And the plans for those wires would come about as a result of the franchising process.
The only infrastructure the Operator & Development Partner will be allowed to work on is the Core Valley lines, IF (and it's still an if) NR and DfT agree to transfer the Core VL to Welsh Govt, as surprisingly that still hasn't officially happened. Any other enhancements will be up to Network Rail and the DfT, because as has been said, almost the entire budget for VL electrification will be swallowed up by the Core VL project.
This is the thing. The problem you indicate would cost money to solve, and the problem itself would act as a dampener on the benefits of the whole scheme. When the government plans and awards the franchise, they have to account for these costs. When the money and contracts are being decided, there's no room for ambiguity. If 24tph or 10tph have to turn back in both directions in central Cardiff, there will be an infrastructure cost to be accounted for.
When I look at the plans, I can't see any way there would be a better business case for a split than there would be for 750V DC wires to go up to Radyr, Penarth and Barry Island. Any city centre turnback capacity will have an upfront infrastructure cost - the real upfront cost of the wires is therefore only the difference between its costs and the turnback costs. Turnbacks in city centre locations are rarely cheap, which is why services tend to be extended through to the other side. From this small-ish capital cost gap, there's then a big operational saving from using LR trains rather than HR.
So if it's confirmed unequivocally that the plans are for a network split, with everything from the Valleys, the Vale, Barry, Penarth & the City line terminating in or around Central station, what would you say to that?
Because whilst I have no inside info, I have read what must be every single news article on the Metro/Valley lines over the past few years, and I can absolutely say from reading everything there is to read on the subject, that TfW want a network split to hive off the Core VL from everything else, no matter what the effect on operational costs or flexibility at Cardiff Central.