AhsanAllahu ilaykum is ignorance an excuse for someone who doesn’t pray even tho he knows that the salah is wajib and is he considered a Muslim if we take the opinion that it is kufr Akbar?

We can not apply the ruling to the person by name this is not for you and I so the asl is that he is a Muslim yes. The people of authority and knowledge are the ones who do so like the official courts mufti etc