Often times shunts are not found using ultrasounds...so not being found using this method is not unusual. Her food did not cause the shunt. It was always present...some dogs are just asympomatic.
Many many yorkies have the surgery completed, yes it is intensive...but they do on to live long healthy lives with completely normal bat results afterward.
Here's more information from the University of Tennessee...Dr Tobias is considered an expert in this field...
Portosystemic Shunts FAQ
Best wishes for Emma...