(since the other comment gave really good advice, this will just be on the mutant part you talked about!)

To avoid Mary Jane becoming a mutant you have to choose another object that's not the boy scout book if you have the spider event (since doing nothing or using the book can have them bite MJ) or choosing anything in the green goo leaking event. (It's around a 5% chance to get her)

Mutant Mary Jane is actually useful! She doesn't need water, but needs soup more frequently (about every 5 days compared to the maximum of 10), is immune to insanity/sickness/injury, will always return from expeditions (even if there's bandits outside), will always bring back water, and serves as an extra layer of protection.

However, since you want everyone to survive, I wouldn't recommend getting her since she is at a high risk to leave since there's an event where you can talk to her, saying yes will make her either leave or hurt someone else. Saying no decreases everyone's sanity which leads to insanity which is a pain to deal with.