I am taking bus to Huanglong from Jiuzhaigou and to Songpan after visiting HL.

Usually, visitors can put the luggage on the bus and the head for HL for visiting, so you can do so. It is safe and no need to worry.

Do you mean that the public bus leaving at 7AM from Jiuzhaigou (or from the Airport, heading to JZG) will wait until you are done sight seeing?

I only know the buses from airport to JZG via Huanglong will stop outside Huanglong waiting for customer's sightseeing. Then take the same passengers to go to JZG.

If you use the buses from JZG to Huanglou and then back to JZG, the buses will also stop outside Huanglong waiting for you.

For other circumstance, I am not sure. Sorry for that.