First, I can't tell you or your husband what to do about exchanging dogs if something is wrong with Silas. That's totally a decision for the two of you.
With that said, if you do keep Silas, he'll be fine in his new surroundings. Those "shock" collars don't really give out too much of a bite, trust me, I've tried them out on myself. And Sibes have very thick fur, especially around their necks, so don't worry about it.
The deal with a lot of people coming around is not a problem. More than likely, he'll be happy more humans show up. The only problem is with the German Shepard, the way it acts will definitely influence the way Silas will act. If the GS doesn't like people coming around, and gets aggressive, there is a strong possibility that Silas will act the same way, when the GS is around anyway.
Other than that, I wouldn't worry about it in the least.
btw, good luck on your schooling!
EDIT:
Forget to add something. If the yard isn't fenced in, you'll definitely need to get a runner. You can't trust a Sibe off leash, especially with other dogs around. Hopefully your parents have two trees in their backyard in order to attach a runner.