name changed as in a fairly unique position for this one - ex police and live in St Mellons. It's not really doubling back - from Porthcawl to Llanderyn (the place in Cardiff where they dropped one person off) to St Mellons to Newport is a direct route West - East. So they just made 1 trip from Newport to Porthcawl then seemed to be driving back. This could be the reason for the delayed ANPR - the most logical route from Porthcawl to Newport is just staying on the M4, if they didn't know they'd dropped someone off in East Cardiff on the way they wouldn't immediately look at the A48. Once you know that, the route they took from Llanderyn to Newport is the main/most obvious way to go, but there's no reason for them to have crashed in St Mellons rather than anywhere else along that route iyswim?
In relation to the poster querying why someone else found it 'scary' - maybe scary isn't the exact word but, having driven past the crash site on Sat and Sun with no sign of anything it is a bit creepy/unnerving/sad to think there were at least 2 people still alive but seriously injured and others dead, just feet away.
It's a pretty busy road, even in the middle of the night as it's the main route for anyone leaving Cardiff City centre to drop off anywhere eastwards then get on the motorway. It's really unlucky, it could have happened a minute earlier/later and there might have been lots of witnesses.