...but I'd have to wonder how the single-nacelle manages to propel the ship without a neighboring nacelle to with which to interact and form an asymmetric field.
Well, perhaps they're ignoring that technobabble altogether.
Yeah, maybe they are. Why they would feel the need to is the question. If it enhances the story, then disregard whatever technobabble necessary. The only thing a single-nacelle ship does for the story, as far as I can tell, is say "Look, this ship is old!" or "This ship is from the relative past!"