Anyways, to test for the gene simply breed the good feather quality birds the poor feather quality birds. If any come out with feather shredding, the birds carry the gene.
Considering that two out of the five don't have feather shredding, I doubt you'll find two that don't carry the gene.
Correct. Since they are probably related it is likely they'll carry the gene.
For the selling question... well, in truth, I've never seen a lavender Orpington with good feather quality so I'm starting to question their very existence.