I've noticed a certain (probably) sound change being illustrated by 雨 （あめ） and the compounds 霧雨 （きりさめ）and 小雨 (こさめ). It looks like this used to be pronounced さめ in all contexts, but the s here was lenited, and I was wondering if I could find more information on this sound change in Japanese and potential other words where it historically occurred, or where it's displayed in the descendants of those words. It would also be helpful to know whether this lenition occurs before vowels other than a.
https://en.wiktionary.org/wiki/%E9%9B%A8#Japanese has an answer for you in the section "usage notes".
Basically, it is only one of multiple theories that the /s/ was originally a part of the word that was dropped (you use the word 'lenited', which is a different phenomenon). The theory I find more likely is that the /s/ sound filled in the hiatus between the starting vowel of ame and the final vowel of kiri or ko.