Answer

A. The reasons behind expositions are definitely relevant, and in my opinion the rule that one does not derive law from the reason of the verse does not apply to expositions. The reason is that an exposition always relies on the expositor’s reasoning, and therefore it always begins from human reasoning. That is unlike an explicit law in the Torah, where it is said that one does not derive law from its reasons. I discussed this in detail in my article on the fifth root and in the article on the status of reasonings.
B. I have thought about this more than once. This exclusion was not stated only regarding the laws of bailees/guardians but in other contexts as well (such as the laws of oaths, overcharging, and more). In the past I thought of partial explanations, but I did not find anything comprehensive and satisfying.