Water: בירור or מתיר

The sotah drinks the water and is acquitted.  There are two ways to understand this process.  It may function as a בירור that she is innocent and hence she is permitted to her husband or it may be an automatic מתיר.  What is the difference between these two understandings?  If there is room to suspect that even after surviving the water, she is really guilty.  The Mishna in Sotah 22b brings opinions that if there are possibilities of some good deeds allowing the woman to survive even for up to many years after drinking the water. If the drinking of the water merely serves to determine the status of the woman, but doesn’t create an automatic heter why shouldn’t we suspect that she is really guilty, but there is a זכות  that allowed her to remain alive? Tosfos (6b bottom) brings from the Yerushalmi that has a derash that we don’t have to assume that she is still alive because of some זכות.   

The Rambam Laws of Sotah (3:20) brings the law that if the woman has a zechut she gets weak and eventually dies.  This is the opinion of Rebbe in the Mishna (ibid.)  In Law 21 he says that if she survives the water she is permitted, indicating even when she becomes weak when we know she is tameh.  It is clear the Rambam understands the waters serve as a matir to permit the woman even if we know she is tameh.  On the other hand, Tosfos (ibid) says that if we know that she is tameh because she is growing weaker, then she is prohibited.  Clearly, he holds it functions as a birur.