Dark Knight Returns Part 2 questions

What I understood was:

1. The plants absorb power of the sun and Superman took that from them to regain his own strength.
2. It wasn't made clear but it was implied that Superman ripped out Green Arrow's arm. This was hinted twice: the outdoor scene when Batman and Superman in normal clothes were conversing (the keyword "torn" was mentioned by Batman) and again at the Wayne mansion directly from Oliver himself.

Point 2 was similarly vague in the comics. The analogous scene for point 1 in the comics was a bit strange because Superman in the comics was having an inner dialogue with Mother Earth but the idea was the same: earth and plants took in power of the sun and Superman took that from them.

1. This is left quite unclear in the movie, but from looking into the actual source comic, it gets a little clearer from Superman's monologue at that moment, even if still a bit vague:

   I have always loved you...Though I was born a galaxy away...I have always served you...The same power...the sun's power...fuels us both...You hold it here...you store it...I beg you...for a suffering world...release it...Mother...Mother...You are...so generous...You give me...your beautiful jungle...I swear...your adopted son will honor you.

   From this it is hinted that it is indeed the power of the sun stored within the plants, or the nature itself (or to name it right as Clark does, "mother earth"). So he does to some degree rob those things of their life essence, but it is really the power of the sun he takes from those plants. Yet it is left a little bit vague if he can actually always do this or if it really is the conciousness of the earth itself that intentionally "gives" this power to him after his pleads.

   As a little addendum, we can also take a look into an earlier plot outline of the fourth part of the comics series as detailed by Frank Miller in 1985 (and which got changed quite a bit towards the ending, but which still contains the respective scene in pretty much the same way). Here it's also explicitly said to be the power of the sun stored on earth that strengthens him, though, put a little more "scientifically" than his rather metaphysical speech in the finished comic:

   ...He can't reach the sun. But the sun's power is held on earth. Photosynthesis, he thinks. So many need me. He reaches the edge of the crater, touches a patch of earth, then a beautiful jungle flower. The flower wilts, and dies. Then, spreading outward from Clark, the jungle dies, for miles around. Clark rises, looking like a man again...

2. Yes, it is indeed strongly hinted that Superman was respsonsible for the loss of Oliver's arm, even if only hinted. This is alluded to in two scenes. When Bruce and Clark first talk outside of Wayne Manor they discuss Oliver's fate and it seems that Superman had to do something to force Oliver into retirement:

   Clark: You know, we almost had this talk ten years ago, when you wouldn't go along with the deal we made. Everyone else agreed, Diana went back to her people, Hal left the planet...
   Bruce: And Oliver, did he agree?
   Clark: That's not how I wanted it to go.
   Bruce: He was all torn up about it, too.
   Clark: He made it necessary, like...
   Bruce: Like I'm about, too?

   And later, when Oliver speaks to Bruce before the final confrontation, this is reinforced. And in Oliver's dialogue he's clearly looking at and speaking about his arm, as if he wants some retaliation for it:

   Bruce: What is it you want, Oliver?
   Oliver: A piece of him, the schoolboy. I always knew it would come down to the two of you someday, the world's not big enough for the both of you...Doesn't have to be a big piece. Just count me in, for old times' sake. Still hurts when it's cold, lately it's been cold a lot.

   Interesting is, though, that only the second dialogue actually appears in the source comic (in a similar form at least). But this second scene already makes the situation quite clear and the fact that the writers of the movie adaptation took the liberty to introduce the first dialogue (in what largely is a very accurate adaptation), elaborating a bit more on the fact that Superman had to do something to Oliver (and Bruce mentioning the word "torn" clearly in jest, as yurnero already points out in his answer) shows that they picked up exactly that same vibe from Oliver's and Bruce's dialogue in the comic.