1-You have tu save her first from the Don and after that you can bring someone yo the safehouse but it will be more like a roommate.
2- The Milk Farm is an event that hapen when you come and go from new Ark city to the Abandoned Town
if you agreed to work as a thug for Don then you should go back to Cassie and ask her if she knows Don then she will tell you his story, after that you go to sleep and when you go down the stairs to see Cassie she will no longer be there, then you will go to Don's guard and there she will be and this is where her route begins.