Oooooh boy, I know it's been a while, but let me share some of my experience about this wonderful woman. Now I know this is a bit off, but Mama Murphy can help you avoid encounters with other unsavory "nouns" as well, so you should see a max difficulty on average for threatening or convincing her to get off the chems off the bat. She'll help you out, but making her keep her word to not go on the chems for her is really no different then her dying, so you will probably get a Super mutant posse from her that will come attack you because the sight is "dead" even though Murphy will not die from chems. This is how you can be sure that you're ready for a life without the sight.
As for Cait, if you complete her personal adventures, she ends up being no different from Piper in regards to your relationship in getting Murphy off of the chems, just in regards to who Piper is after she first offers to leave Diamond City with you.
Should really watch where you're walking. I went there to impress my chem free Cait, and Murphy was so mad she had to quit because I persuaded her off the chems she sent Super mutants after me when I walked back to my settlement at Red Rocket. It was only a day in game, and I had been there at Red Rocket for a week and a half with nothing going on. Guess she fell for the bad boy. XD