Depp talked about his heavy drinking prior to splitting from Paradis and his drug use with Bettany on the first day of the UK trial. He also talked about both in this trial.
His entourage travelling with him procuring drugs on his behalf was covered in both trials.
His sister's intervention also came up in both trials.
I am not saying his testimony in this case is an exact match to that he gave in the UK case. It would be surprising if it was. But I do not see the contradictions that you see. On the above points, his testimony in the US case appears to be entirely consistent with his testimony in the UK case. The only point Heard's lawyers seem to have challenged regarding his UK evidence is his testimony over how drunk he was on the flight, where what he has said in the current trial is not consistent with what he said in the UK. His response was, in effect, that he had caved in under cross examination in the UK.
Impeachment, as a legal process, applies only to public officials. The other sense is about discrediting the evidence of a witness. His evidence may be impeached, but he will not be impeached personally, nor will he face the threat of impeachment.
I would be amazed if he apologises for his behaviour.