Did you try hitting the '1' or '2' key to see if you could select options that way?
Try going into the Game section of the options, and select 'Dual dialogs' to switch the setting and accept the change, then enter the Game section again and select it again, put it back to its original state, accept, load the save with Leandra almost defeated (if necessary) and see if that helps.
If not, try the option opposite whatever you have it currently.
If that doesn't help, if the dialogue was triggered with the same main character each time you defeated Leandra, maybe try killing that character first, so it will use the other one. Alternately, you might be able to move them far enough away they they are not included in the conversation.
Try dropping the soul forge spell, and see if that will get the game to skip that part of the dialogue.