Even within an architectural firm, the professional title "architect" is not used to describe someone who does not have a state license; there are no unlicensed architects.

If by "signing off" you mean an outside architect would review the drawings but not put his name or stamp on them, then the risk is taken by the designer you hired because his firm is likely to forbid him from doing outside work. The primary concern of the firm is that they might be held liable for their employee's errors and omissions and their insurance provider might be forced to defend the firm.

If the outside architect actually stamps the drawings, then the risk you ask about is taken by that architect since it is a violation of state registration law for him to stamp drawings that were not prepared under his direct supervision and control (i.e. in his office, by his employees; calling someone a consultant doesn't qualify). But this is a violation of state law that is difficult to prove and often not aggressively enforced by state registration boards who don't usually have the budget to do that. If caught, the architect would probably get a fine or be put on probation.

If an architect stamps your drawings, he would be the design professional of record and would be liable for errors and omissions although you would not have a contract with him so you would effectively take that risk yourself.

Since an architect's stamp is rarely required for a permit for a single family house, the best approach is to not stamp the drawings and let the designer use whatever professional advice he wants in order to produce the drawings and avoid using terms like "sign off" or "approve" or even make a record of that process.

Where I am, either an architect or a structural engineer must stamp and sign-off on the plans, and becomes liable for any omissions and errors. They are agreeing by stamping it, that it meets structural codes for the area.

There are no regulations here for who is actually required to draft those plans. I drew my own. It was up to the engineer or architect to review what I drew, calculate loads, footing sizes, beams and support posts, and have me modify the plans to include them prior to his stamp. In the end, ANYONE could draw the plans. It's the engineer/architect who ensures they're structurally sound and remains liable for anything that goes wrong on an engineering-structural standpoint.