The association between a voice and a character is probably based on the character's UUID (but I'm not sure). What you could try is to create a mod in which you copy a generic player character, but change the file name of that character file to the UUID of the origin character whose voice you want to use.
Regarding your second question, that would definitely be hard. The reason is that many interactions with Gods are custom-scripted, so you would have to change those everywhere in the relevant story goals and dialogs. There's no single "race => god" mapping table that you just change.
