You say she's not food motivated, but have you tried some really high value goodies? Boiled chicken cut up into tiny (fingernail sized) pieces, for example? Tiny bites of cheese or low-sodium hot dog? Obviously at her size you need to be mindful of calories, but if you can find a treat she's willing to work for, it'll be a lot easier to work through this.
If you find a treat that motivates her, you can try luring her through the door, but I'd also consider target training her to touch your finger or an object with her nose, like shown in this video:
Once she has a nose touch down, you can work on teaching her to touch something like a sticky note that you can transfer onto the dog door, and encourage her to touch it with more and more pressure until she's going through both directions on her own. Definitely lock or block access to the doggy door while you're gone until she can't get trapped outside - I'm super glad nothing really bad happened the last time, but that's scary to think about!