MacBeth might die full of sound and fury, signifying nothing; his death, brought about by the man whose children he killed, doesn't have any meaning for him

I have to disagree with you here. MacBeth continues to value his life and power, even after all the prophecies come true. When MacDuff shows up the embodyment of them he chooses to fight on. He backs up his violence and ambition with the stubborn determination and courage that saw him lauded at the beginning of the play for his deeds in battle. Although he recognises the wrong in his actions, he is quite happy to live with the results and defy the prophecies if he can.

Londo, on the other hand has realised his errors much more fully as crimes and regrets them. Although he has played the part of the tyrant for the Drakh he always intended to rebel when he could, and has accepted his death so he can use it as a weapon against them. By choice he would not have fought it. It was the compulsion put on him that drove the prophecy to it's conclusion. As you say it is part of his decision to apologise that changes the meaning, but it is different from MacBeth's story in more than just meaning. From the freeing of Narn it is different in intent as well.