The sensible solution, which I have seen done very successfully before, is to add a 1/1.5hr hr on site/skype step, in which the candidate goes over the solution

The sensible solution, which I have seen done very successfully before, is to add a 1/1.5hr hr on site/skype step, in which the candidate goes over the solution, and asks for a small feature. It both proves that the code was written by the candidate, shows some of the personal stuff the homework didn't, and makes it far easier for the candidate to perform well, because they are familiar with the code, as they wrote it! It's not an expensive investment for what you get, and since the meeting should happen soon after the code is written, it's easy to provide basic feedback.