Prohibition [or perhaps, "a prohibited thing"??]
Both. Idea is "relations with him are prohibited".

And the romance-novel of our lives that we haven't finished reading
Is no longer celebrated with a heavenly drum-roll.
"Небесные литавры" ("timpani-drums of heaven" literally) can be for "thunder": http://www.youtube.com/watch?v=cb_W3zRDan0 because of specific sound. I think it is initial metaphor for it.
Also it can be met in context of "music of heaven", which is associated with spiritual uplift and so on. You can remember international idiom "marriages are made in heaven" here.
"removed from the parentheses" is simply "removed from area of application of...".

How can I not regret [the fact that I know] you?
Blue text is wrong. Just "regret you". Feeling are directed to him, not facts about him.

From this [above-mentioned sense of sadness], the sunset-colored copper
Pours with molten alarm into my soul; [Again, I think I understand the literal meaning but not the idiom!!!]
"sunset copper" is about color of sunset, yes. Just "looking at sunset she feels worry.

I'm hoping, again and again,
That you're my impossible [source of] joy;
There is no joint here. These are different sentenses. Just "You are my...".

[Not sure if this means "Connecting us" or "Separating us"!!!]
Connecting.

[or, possibly, "My source of peace"?]
No. "World" is correct.