The more I think about it, the more I just CAN'T see it ending well no matter what you do.
Jean: "So, I see how Scott, Hank, Prof. X, Bobby and Warren are doing, but where am I?"
"Oh, you're uh, dead?"
"How did I die?"
"Magneto killed you. Only, it wasn't actually Magneto. It was Xorn. Except it wasn't actually Xorn, it was Xorne. Maybe."
"Can I at least go and see my family?"
"Yeaaaaaaaaaaaaah. About that."
Like, seriously, if Dark Phoenix wasn't a problem BEFORE, when Jean was a relatively innocent girl, imagine how dangerous she would be knowing that at some inevitable date in the future she and her entire family would be slaughtered and none of the people who loved her did anything about it. Not Scott, not Hank, not anyone.
There just is no way that bringing teenage Jean to the present shouldn't be catastrophic.