1) Basically, no.

"Don't take this heaven from one
If you must cling to someone
Now and forever
Let it be me"

 First, the one you use in the first line is very ambiguous, if not completely wrong. A correct usage of one is seen in this sentence: “The older one grows the more one likes indecency.” (haha)
However, your last line, let it be me, makes it clear the someone refers to you.

2) Thats a bit harder....some rules in English are poorly defined, and this may be one of them. I'll look at my style and grammar books later and see if I can find a definite answer.

3) They usually CANNOT be used interchangeably. Take the sentence I used before,
a) “The older one grows the more one likes indecency.”
b) “The older someone grows the more someone likes indecency.” If you do this, people listening/reading will notice the awkward usage and may think that someone is a specific person.
The way to make it work:
c) “The older someone grows the more they like indecency.” (notice the removal of the s off "likes")

But getting back to the topic...
If you want the poem/lyrics to clearly be understood as referring to you:

"Don't take this heaven from me
If you must cling to someone
Now and forever
Let it be I"    (you could say let it be me, but it could be repetitive.)

Hope I helped!