I think if you continue to use the chicks that look like they have birchen down color you are going to end up with leakage in your lavenders.
Extended black chicks have white underbellies, chicks that are birchen or carrying birchen are generally all black.
If you go to the kip calculator and use a bird that is half extended black and half birchen E/E^R and cross them you get birds with leakage.
On the lavender chicks, I think it is ok to use the lighter birds, but remember to build the house before you paint it. You could possibly be getting rid of some nice birds that are just off a bit in the color department.
On the split lav chicks popping up, it's that the odds are against you at this time.