I am a newbie to the breed. My question: If sheep are not available, how can they herd ?

If sheep are not available, you will find that PON tend to herd their human families.

 There is a site which offers alternative remedies for conditions like Parvo,etc. Is the site owner a Holistic vet ? Didn't they call such remedies "snake oil" in days gone by ?

 Consult your family vet for all health conditions. Don't take chances because you want to please someone. Think for yourself.

My PON has an underbite, can I show her ?

Only if the judge is very nearsighted because this is considered a fault.

My PON was spayed but I would still like to have her participate in useful activities. What do you suggest ?

Consider therapy and obedience training