Well I did have a theory that the Ultimate THOR hammer would go to Jane and THOR would reclaim Mjolnir, but Mangog has literally shattered that idea.
For Jane to receive a miracle cure at this point seems unlikely unless the cosmic cube, time gem or reality gem are used to restore her.
However even if she does perish that does not mean that Mjolnir will return to THOR.
My money is still on Jane becoming a Valkyrie Thor and retain the hammer. Real Thor probably getting a new hammer which we will find out was the real mjolnir. Also guessing it will limit the powers in the mold of The Ultimates and movie hammer. Given the underwhelming display of the War Thor it also would not surprise me to see Volstagg pick up mjolnir transform and die. Jane Foster might die as well and still be Valkyie Thor.