The gemara states that the bag of chametz which waw deposited by Rabi Yochonon Chakika was pierced by mice which were spreading chometz all around on Erev Pesach. Question: Why is this detail relevant ? Wouldn't Rebbi give the same spak if mice had not opened the chamets bag on Erev Pesach? Yet, Rashi makes a point ( DH Venokvu Achborim) that the bag was diminishing ( from its content) and on the 14 of Nissan the price of chametz goes down. Why do we need both devaluations: 1) from the content going down and 2) from the price going down? Wouldn't any of the two be sufficient to sell the bag?
(a) The intention of the Gemara is that Rebbi was concerned for the Mafkid's loss , as you mentioned in your second question (and not necessarily that the mice would spread the Chametz around). From here Rashi inferred that the Chametz was losing its worth because of two reasons: 1. Pesach was quickly approaching, and 2. the mice were diminishing the quantity of Chametz that had been entrusted to the Nifkad. The Yefei Einayim points this out as well.
(b) The reason that the Gemara needed to mention these two concerns was because of the Halachah that a Nifkad may not sell an item entrusted to him unless it was losing value more than its standard, expected loss (according to the Tana Kama who argues with Raban Shimon ben Gamliel). The case of our Sugya is when it was not decreasing in value to that extent, and therefore Yochanan Chakoka'ah would not have asked about the Chametz if it were only because of the loss due to the mice alone (because we know the Shi'ur at which one may sell the item to prevent further loss to the Mafkid). And if he was asking because of the loss that Pesach was going to cause, he would not have come to ask so early before Pesach (the first hour of Erev Pesach), since the Chametz was not going to become Asur until the sixth hour.
Now that there were two causes of loss of value, Yochanan Chakoka'ah thought that he should sell the Chametz before it incurs its expected loss of value, since Chametz was selling for very little and if he were to wait until the sixth hour the Mafkid's loss would be so great that it would equal a loss of more than its expected decrease in value. The Gemara teaches that even so, he may not sell it.