Hey Peter!
Were you still having issues with this? Just wanted to suggest considering using an app for handling back-order products. We just launched one called Early Bird which makes setting up back-orders for your entire catalog super easy.
It also means no theme modifications or liquid code (one-click setup, removed completely on uninstall).
You can choose to activate back-orders for your entire catalog (even 4000 products!), or only for specific products you choose, and it will show clear messaging/labels that it’s a back-order once the stock reaches 0. And you can of course edit this messaging/styles as needed.
Feel free to check it out on our Free plan to see if it works for you! I’d be happy to help get you set up too if you have any questions or issues.
Here’s an example product with some of the different options we offer (password is “demo”). Hope it helps!