You shouldn't need any console commands for Sylgjia as she is already on the marriage list. She is currently taking care of my house in Whiterun. As long as you are wearing the amulet and have delivered the letters to her family in Darkwater Crossing, she should talk to you about marriage. Do you have an earlier save you can maybe go back to and try again (before you tried to change your relationship settings)?
Yes, but I just got out of blackreach, I have the annoying (especially now) habit of just over writing my last save in dungeons, so I'll loose a few hours.
Oh and I didn't mess with the addfaction command as she is already available I just increased her dispostion towards me, as this seems to be a prerequisite for marriage as well.