I'm assuming Wales had their own copy of Eastenders seeing as they were the last to start tonight's ep.
So Wales and N.I. both had a copy of Eastenders, so at 8:05pm both could have played it as planned. This would have given Wales thinking time till 8:30 to work out what to show at 8:30pm as they could not play Panorama directly themselves or opt into the N.I. feed showing it
You said yourself that you
they had their own copy of EastEnders.... the fact the they didn't play it at the scheduled time would seem indicate they didn't otherwise they would... Occams Razor
Taking what Northern Ireland was doing, as has been explained, isn't as easy as you suggest. Technically it's not that difficult, but it needs to be arranged and set up.... and that's probably going to be low priority compared to what's going on in network BBC1
Finally you assume that Belfast knew right from the start of the incident exactly what they were going to do. Both them and Cardiff were almost certainly doing the same things and having the same thought processes in that 20 or so minutes they had between realising how long the breakdown was and the end of their local programmes.
Could well be that one got lucky and the other didn't
As had been said, it's all very easy suggesting what should have happened hours after the event from your bedroom