When you interacted with your bike shop about this issue, did they bother to check the spoke tensions? If so, I'm curious about what they found.

Safe to ride? Depends on what safety means to you. On the rear wheel, if a spoke pulls through, you will feel it through the wobble that develops; however just a single spoke won't throw you off the bike. Once a single spoke goes, though, others will probably follow making the wobble so severe that the bike will be un-ridable.

When you get the new wheel, ask the bike shop to check the tensions.