MB 10: If he did - what he means is that in a case where one has already said these words, and only then does he worry that he said the "Lema'an" after the "Uchtavtam" of the first paragraph, he does not have to go back - we assume that he said according to habit, and therefore said "Lema'an yirbu" after the "Uchtavtam" in the second paragraph. All the more so if one finds himself at any other point in the middle of a paragraph [and does not recall what he read up to this point], we assume that he said all of what was beforehand without skipping a verse or a word.
MB 11: There is no need to go back - All this is if he is praying alone. However, if he is praying with a minyan and he sees that the chazan is at the "Uchtavtam" of the first paragraph or near it, he should go back to the "Uchtavtam" of the first paragraph, because we can see that he was mistaken and his continuation wasn't correct. The same is true in the other direction: if his situation is such that he would ordinarily be required to go back to the first "Uchtavtam", but he sees that the chazan is at or near the second "Uchtavtam", he should also continue with "Lema'an yirbu", since we assume that he read the second paragraph like the rest of the minyan. We will discuss the laws of one who accidentally changed a blessing from what our sages set down (which the Magen Avraham discusses in the name of the Ramba"m) G-d willing in siman 68 in the Mishna Berura, see there.