I wouldn't stop giving heartworm preventatives too quickly.
If my dog had a shunt and was stable, I personally would give it.
You may be able to give it less often though (like every 42 days), so that's something to talk with your vet about. Flea preventative is a bit more optional unless you are worried about ticks and lymes.
Vaccines are a hard area.
Has he had all of his puppy vaccines?
If yes, then I wouldn't worry about it until you get a diagnosis.
they should not be given yearly either.
If you are in NY and are going to a specialty hospital, Cornell is one of the best with shunts.
Ultrasound is only 60-80% accurate at detecting shunts, so it isn't surprising that the vet didn't see anything. Was a Protein C test done?
All Yorkies over 20 weeks really should be bile acids tested since the problem is so bad in the breed. Then you wouldn't have to wonder either...