First, I would like to clarify that I am not a lawyer, just another registered engineer like yourself. With that huge caveat out of the way, I'll offer what advice I can give based on 20+ years of experience.
The initial responsibility lies with the person who has stamped the drawings. By affixing their PE stamp, they assume responsibility for the contents of the design. This is where incorporation becomes important for liability purposes.
However, you'll notice I said "initial" responsibility. As registered engineers, we all carry a certain amount of responsibility for what we do. If I negligently act on a design, I would also be held responsible in addition to the engineer who stamped the drawings.
Finally, you have to accept the fact that, at least in the US, people here are lawsuit crazy...they'll sue for anything and anyone. If your firm is sued, chances are you'll be dragged into court whether you're personally at fault or not.
It's not uncommon for the principles of a small firm to give direction to a staff engineer on specific projects. I would say that the level of involvement is directly proportional to the size of the firm (i.e. the bigger the firm, the more company business the principles will be doing versus engineering). The interest that they are showing in your particular project could be interpreted in two ways. First, they may just want to stay involved with the design process since they are engineers at heart. Second, they may not have enough confidence in you to trust you with the design. That could be a problem on their part (i.e. not having enough faith in their own company) or on yours (i.e. maybe you're not performing up to their standards). If you feel that your firm's practice is questionable, or you don't feel comfortable with how you're being treated, then talk to your boss. If you suspect there are some questionable engineering practices going on, then find another firm.