My first thought was 'of course he knew' but I've changed my mind. He's toast of course, but if he was liked he'd easily ride this out. he's not, so everyone immediately thinks 'oh, yeah. right, you useless blancmange'. But he's a lawyer and a bloody good one, and he needn't have used the words he did, unless he believed them. If he'd known Mandelson had 'failed' DV he'd never have chosen his words as he did. So I believe him. He really wasn't told, for whatever reason. If Mandelson hadn't been outed further late last year, he'd still be Ambassador and no one would give a shit about this DV failure (it's not a bar to getting the job it's a risk factor that can be recorded and overridden). Nothing illegal happened in making the appointment.
Starmer was finished already, and this is just another unnecessary nail in his coffin, but I don't believe this should be the reason he resigns. The fact he's a useless PM at home is the reason he should resign, when there is a sensible replacement available, in an orderly fashion. We don't need more chaos. He should absolutely stay in place until Trump is done and the clock is ticking there and it will happen soon. If not, then Starmer will be gone by the end of summer anyway.