At least your WG fetish is completely out in the open now. Sounds like she's been fully aware of it for some time, maybe a long time given how big your ex was.
That might mean you can bring it up with less fear of judgement in future; maybe she would be up for role play or WG dirty talk to help satisfy it. She mentions being between a rock and a hard place; role play etc could be a good compromise - they would allow her to satisfy you without there being pressure to actually gain weight or stay fat. I.e. the fetish stuff would be contained and she could live healthily the rest of the time without worrying she's letting you down.
In my experience, when my partner knows about my fetish, it acts a demotivator to losing weight, even when I'm not encouraging them in any way to gain and when they want to get healthier.
It's hard work losing weight, especially as you get older. Hard enough for most people as it is, let alone when you know your partner prefers you fat.
So I would definitely recommend doing all you can to help support her with this. Losing weight could not only make her happier but also more up for being active with you going out and doing cool things together, as a family.