Hmm, definitely a puzzle there!
Assuming you are correct about the parentage, here are some thoughts that may or may not help:
I could almost believe I see a bit of a crest on the Bielefelder (little puffs of feathers on the sides of the comb), but I don't see it in either of his sons, so I don't know what to think there. If the Bielefelder had a crest, it could be passed to his son and then on to his grandson-- but I would expect to see it on the son, not just the Bielefelder and his grandson.
Given that Bielefelders do have the same coloring at Crested Cream Legbars (autosexing), I wonder if the hatchery had a mixup at some point, and a Crested Cream Legbar got crossed into their Bielefelder line? (If any pullets from white-egg or brown-egg hens, sired by the Bielefelder or his sons, lay blue or green eggs, that would be a pretty strong indication of Cream Legbar in there somewhere. But it would also be possible to get the crest and not the egg color if a cross happened several generations ago.)
I have noticed that some Crested Cream Legbar roosters do not seem to have visible crests, even when they have the crest gene, so I suppose that might be what happened to one or both of the mixed-breed roosters. But that doesn't explain how the white chick got such a big, obvious crest. (Maybe some kind of modifier gene is involved to control the size of the crest?)
The comb also reminds me of what the combs look like on Crested Cream Legbars. They have single combs, but the crest pushes the comb a bit forward, so it crinkles instead of being straight. So no matter where the crest came from, that might be an explanation for the odd-looking comb from two single-comb parents.
The fact that your odd colored chicks came from breeding a son to his mother makes me suspect that one or more recessive genes are involved in that.
The white color might come from the recessive white gene. That "should" not be in either Bielefelders or Fayoumis, but in this case I wonder if the Fayoumi hen does have it. If she does, and if one of her sons inherited it, then breeding that son back to her would give about 25% white chicks.
(If I knew of a recessive gene for a crest, I'd be suspecting the Fayoumi of having that too. But I have only heard of crest being caused by a dominant gene, which is why I was looking hard for possible crests in the parents.)
