When I try to preview my offline donation form, the preview displays our homepage instead of the donation form.
The reason you are seeing your home page instead is because offline donation forms do not have an existing frontend page, so there is nothing to preview. If you would like to preview what your donation form would look like on the frontend, you can temporarily change your form to online/offline or online-only.
In the donation form settings, navigate to step 1 Identify Donation Form.
Click the dropdown menu under 6. Interaction Mode and select online-only or online/offline.