I have to agree with #2. I may offer a somewhat simplistic analysis.
As the metaphor goes, coal under pressure becomes diamonds. I think that she is saying that a black person does suffer, but that suffering makes a better person. This turns the coal into a diamond. It is a nice idea.
is the total black, being spoken from the earth's inside.
To me, this means that all of us should make lemonade from lemons. Even if you have not suffered as much as a group or person, you can overcome whatever challenges you face by making the best of them.
Poets do this. They turn troubles and tough experiences into beauty.
I have never been able to understand this poem as I think it was probably meant. Knowing what I do of Lorde, it seems likely that she is trying to make some sort of racial statement, particularly in the last few lines. However, I cannot grasp it.
For me, the meat of the poem is the long middle stanza. I think that its message is about the power of words (and I wonder if part of the racial point is the power of racist words to hurt). Lorde is telling us that words can have very powerful meanings and very important impacts. This is, to me, a statement of why she thinks poetry and the use of words is important.