Single combs are recessive. The fact that you got single combs out of your GLW roo means he carries only one rose comb gene and one single comb gene. Single combs I believe are a DQ if you were showing them... You don't by any chance have any black orpingtons though, do you??? I would kill for some eggs from your GLW roo over black orps for a project i want to work on... pleasepleaseplease say yes! Slim to none chance of that, I know...
I don't know about the dark chick. I would think that would be from the father. I would be more surprised that ALL of the chicks got single combs. Only half should have, statistically, but that is a small number, so anything could happen. Are you POSITIVE that is the only roo that could have gotten together with your buff hens?
Edit to add: on the second group, I'm surprised that the buffs in the second group did not get all yellow legs, which I believe is recessive and your GLW hens should be homozygous for. (or "for which your GLW hens should be homozygous" in case the grammar police are on duty right now)