r.e. CB400n head cover bolts from Silvers

Looking at the website I would say what they sent you was a genuine Honda part. In that respect warranty would really only apply if the part was fitted in a Honda workshop by a trained mechanic. In a similar situation I also would just have sent you a replacement foc, any extra problems that arise from such a situation are not covered and in fact there is no obligation to even replace the part free of charge. Can you prove the part was faulty, can you prove your torque wrench is properly calibrated and as for the Haynes Manual, well its not the factory manual and are you sure the figure was correct. I don't wish to cast an doubts on your ability as a mechanic but I assume you're not a trained Honda technician ( actually I hate that term) or a qualified motorcycle technician.
I'm sorry if I sound a little biased, but in my experience Silvers sell genuine parts or good quality aftermarket, oem type items, not junk. I will admit that I have always been on good terms with the people there and had a lot of contact with them over the years in my work, not even there not being the job I hoped for there would turn me against them.
As it happens these bolts always seemed very prone to doing such things, the one's on mine were replaced with a stud and nut by a previous owner and I have seen no reason to do away with that. I'm just a parts idiot, even if it's over two years now since I had anything to do with it it's all I've ever done, but I always thought that a stud and nut would have been a better idea for holding the rocker cover on than Honda's way.