can forms be filled out off-line and then auto synced/uploaded once there is an internet connection again?

You can try to download a fillable PDF of your form where your user can use to fill it up offline and save. Some PDF Readers allow you to save fillable forms such as Adobe Reader.

Then you can just submit the PDF form when there is an internet connection. For more information about fillable PDF forms, you can check this article: How to create a PDF form.