I'm very unsure of what you're arguing with me about. I am simply stating the following: all chicks from a lavender father and any of your possible mothers would be black. This also applies to chick down. If you do not trust me, look at the genetics calculator:
Your other rooster is blue barred. That means 25% of offspring are single copy barred, 25% black, 25% single copy blue barred, and 25% are blue. I know you are adamant that chick down does not correlate to adult color, but in fact it does. The grey chicks are not black, they are blue meaning they at the very least do not have the lavender father. If the Faverolles is certainly the mother, then you can be absolutely certain the Sapphire Gem is the father. Chick down is a great tool for figuring out the E locus bases in chicks and chick down does correlate to adult color. Yes, mixed breeds with uncertain genetics can make guessing more difficult but it's still an extremely valuable tool!