No real problem riding an empty trailercycle at all. There is just one issue: if you ride on rough roads, the trailercycle might jump on those bumps. One suggestion if you ride on rough roads: install a rear rack on the trailercycle too and put a pannier there after you've dropped the kid.
As for riding in traffic? I would do 1-2 rides with the child on quieter streets at first, to make sure he doesn't step down at intersections or when crawling up a hill. But once you have cleared those issues, use the tandem or the trailercycle on any kind of street or road you are comfortable with.
As for riding with empty seat(s): I have ridden the road train (tandem + trailercycle + cargo trailer, or single bike + trailercycle + child trailer) with 3, 2 or 1 persons. And I even did half of a tour riding solo the tandem + cargo trailer + trailercycle on top of it. And my top speed has been around 60 km/h. Usually, all it takes is 10 seconds to adapt to the new weight distribution. As for curves, you need a bit more space, but I found this is a problem only in bike-specific "installations". Even with the whole road train, I'm still more flexible than a single car.
So should you go for a trailercycle or a real tandem? A tandem handles better and will last forever, but you'll need more cash. Questions to help you decide:
– What about parking at pre-school and at your office? If your office parking is vulnerable, you might be better with the single. And if there is parking at pre-school, you might leave a trailercycle there.
– What distance will you need to ride solo?
– Will you need to bring the son in years afterwards or will the elementary school be close to home (or with school buses). A trailercycle will not work once the child is older than 8 to 10 years old.