The simple understanding of the Gemara is that לפני עור only applies to the מלוה and the לוה and that they are always עובר. Tosafos points out that the ערב and עדים can also be עובר if the loan would not take place without them.
The questions can be asked, why are the מלוה and the לוה always עובר? Let's take the following case. Reuven wants to lend money בריבית. Both Shimon and Levi want to borrow. Reuven decides to lend to Shimon. Why should Shimon be עובר לפני עור, after all, if Shimon didn't borrow the money Levi would have? Why isn't this a classic case of חד עברא דנהר where there is no לפני עור?
The ( משנה למלך (הל' מלוה ולוה ד:ב quotes the פני משה who says exactly this. If there were other borrowers available then there is no לפני עור on the לוה. However the משנה למלך disagrees and says the following סברה.
The only time there is a heter of חד עברא דנהר is when without you, the other person could do the עבירה with no help. In the classic case of נזיר, without you he could go and get the wine himself. Since he can violate the עבירה with no help from a Jew, your helping him does not violate לפני עור. However, by ריבית, the only way the מלוה can violate the עבירה is if a Jew helps him. The מלוה needs a Jewish לוה. Therefore, even if there is another Jewish לוה, since, bottom line, a Jew will be מכשיל him, there is an איסור of לפני עור. The fact that if Shimon won't borrow the money Levi will, is irrelevant, the bottom line is that for him to violate the עבירה he needs the help of a Jew (and מאי נפקא מינא whether it is Shimon or Levi, someone is being מכשיל him)and therefore there is always לפני עור by the לוה.