If an abaya isn’t see-through when worn, but the body shape can be seen through the fabric when it’s held up to the light, is it still permissible? As a woman, I struggle with this a lot, especially in the heat, and because of it I wear multiple layers when I pray or go out. Is that necessary?

You sound like somebody who suffers from OCD. You can let your mahrams look at you and tell you if your shape actually shows or not. If they say it doesn't show, then close the subject altogether. If they tell you it shows, then just change your abaya rather than wearing multiple layers. Just wear something that is looser, and that's it.