PS: I guess I'd particularly like to know the differences between Simple, GM Voice, and GM+Speaker Voice. I understand that they each display onscreen in a slightly different format, that much is apparent, but I feel like I'm still missing something.

The main thing to get in convs is that the blue line represents the other person and the red one you. So, when starting to build one, you should generally leave the 2 fisrt lines empty. No idea why, but if you start with 1st line it breaks things up when doing multi branches.

So, by default the NPC portrait will appear properly if he "speaks in blue".

You may adjust that using the menu "speaker" thats allows you to change the portrait of the person talking, thus making several chars appear in a single conv possible.