Its Taika. He is mostly going to play Jane as Thor for laughs. Feige wants this for woke reasons. So I wouldnt worry too much about the why or how of Jane wielding Mjolnir. I doubt Taika or Feige are gonna let the why or how of it get in their way. Its barely an inconvenience.

Theyll cram some abbreviated "origin" into the film and transform Jane from the dorky egghead into a warrior that smashes faces and breaks bones with Mjolnir.
If they really wanted to do this they should have developed it across a trilogy. Reintroduce Jane in Thor 4 and move her towards becoming Thor and Thor towards becoming the All Father over the course of the trilogy. Trying to cram a female Thor origin story, Jane becoming a different character, and Thor going from the "dude" to the All Father is probably going to feel underdeveloped.
And I didnt immediately think of it, but its ridiculous that they arent giving Mjolnir to Valkyrie. Her origin and redemption in Ragnarok, and her actions in Endgame, and her becoming Queen made her the obvious female to wield Mjolnir.
#subvertedexpectations