What do you think is the differences in souls?

[Cleretic]

I'm curious to know what happens to the original soul when an Ascian takes over someone's body, though. Do the two souls merge and share their memories, or does the original soul just get overridden unless it's of comparable aetheric density?

Well, the most present example of this we've got is Thancred and Lahabrea; Lahabrea stole his body, but it seemed like Thancred was pretty okay afterwards. That seems pretty clearly 'cram 'em down and override everything'. As to whether they had access to the host's memories... I guess the question there is 'how good do you think Lahabrea is at improv?'