The template sensor should filter out 0 values. As long as your source sensor dropped directly to zero it should prevent this occuring again. Look closely at the history of the last incident did it drop directly from 24806 to 0 or was there a point recorded in between?
If so you will have to make the availability template > that intermediate number.
No there is no way to import your old energy history.
You don’t need a utility meter. You can feed it directly to the energy dashboard.