Should I zero out all entries that appear in a Releve 1 that is automatically generated for Ontario? It seems to have auto-populated some numbers from the T4 for Ontario.

If you didn't receive a Releve 1 with your T4 slip, then you don't need to enter a Releve 1 slip. Click the back button, and click the "Save T4 & Skip Releve 1" button to continue without entering a Releve 1 slip.