I will answer last question first, you can't change your choice once you choose it even you start to play this game again. Unless you have remove persistent file in %appdata%>renpy>lustfuldesire... and in your save folder. Because your choice has been save in these files.
For other choice, I'm not sure but I think if you choose other choice except 'to kill nameless' and 'to serve nameless' and you don't get the witch's mind protection, nameless will see you as neutral. If you choose 'to kill nameless', nameless will see you as enemy. But I don't know an outcome if you choose 'to serve Nameless' too.
BTW please wait for another comment to be sure.