Even though I’ve been a SU user for almost 15 years, I still run into this problem (if I’m not thinking) when I explode a group. I’ve lodged a Feature Request to change the default behaviour so that Explode always assigns exploded raw geometry to Layer0, here:
https://forums.sketchup.com/t/explode-to-layer0/96864
Good luck with your remaining work: Push/Pull is very intuitive and you can easily apply an appropriate style to your final render to make it look nice… so you might not be in as deep s__t as it feels like!