Two simple questions on a sentence

A. This scar was the only hint of Harry’s very mysterious past, of the reason he had been left on the Dursleys’ doorstep eleven years before.

[The situation is that Harry, the main character of this novel, has a scar on his forehead that makes him a special wizard because it is to do with Harry's very mysterious past and because of the past event, he lost his parents and got the scar, so he lives with the Dursley family.]

Q1) What does "of the reason he had been left on the Dursleys’ doorstep eleven years before" grammatically modify in sentence A?

1. the only hint

2. the only hint of Harry’s very mysterious past

Q2) Must there be a comma before "of the reason", so that "of the reason he had been left on the Dursleys’ doorstep eleven years before" shouldn't be seen as grammatically modifying "Harry’s very mysterious past"?

I would say 2 to the first question and yes to the second question, but I'm not sure whether I'm correct.

Would you please help me out?

If my answer to the first question is wrong,

can't we think of "of the reason.." as modifying 2? Because, to me, the structure looks the same as "I got a message about that from him". Here, "from him" modifies the whole "a message about that", not only "a message". In this way, I think the whole thing of "the only hint of Harry's ...past" is being modified by "of the reason..."