According to רש"י the case of דארווח לה זמנא is where the מקדש (is the מלוה and he) extends to her the due date of the loan. It is considered only הערמת רבית, because there was no agreed upon רבית at the time of the loan and he did not receive anything (tangible) from her. תוספות disagrees and maintains that in such a case it would be considered (רבית גמור (מדרבנן since she is saving him the פרוטה that otherwise he would be obligated to give her.
According to תוספות the case of ארווח לה זמנא is where a third party is influencing the מלוה to extend the loan by giving him a פרוטה. There is no רבית at all here since the לוה is giving nothing to the מלוה. It is, nevertheless considered הערמת רבית, since it is considered as if she is giving the מקדש something and he in turn is giving something to the מלוה; it appears as if the מקדש is acting as an agent for the woman to give the מלוה something extra.
He did not return it; he did not fulfill his obligation - לא החזירו לא יצא
The requirement of a תנאי כפול is suspended in cases where the intent of the stipulator is clear that the stipulation must be fulfilled otherwise there is no agreement. A case of תנאי ומעשה בדבר אחד invalidates the תנאי (only according to some opinions, or) if the two contradict each other (if we maintain that ע"מ is לאו כמעכשיו דמי), otherwise (if we maintain ע"מ is כמעכשיו דמי) then even תנאי ומעשה בד"א is valid.
And if not, he did not fulfill his obligation - ואם לאו לא יצא
If a donor is giving an item to many recipients in a מתנה ע"מ להחזיר situation it is not necessary for each recipient to return the item (in order for his מתנה to be valid); it is sufficient if the item is returned after all have used it for its intended purpose.