on the first question
well, see both lavender and porcelain are expressing lavender visually, so every one of those would be lavender chicks, they would also be split to mottled (as mottling is recessive also)
So all your F1 chicks will be lavender split mottled. Now if you back breed these, you will get both mottled and split mottled chicks. The result will be
lavender, lavender mottled, porcelain, lavender based buff columbians, and possibly a few lavender based duckwings. The mottled will be obvious, the non mottled colors should be splits for mottling.
I have heard of this so called lethal gene in lavenders. I personally have bred lavender to about every pattern I have and in various lavender to lavender combos. I've never once had any random deaths from any sort of lethal gene...Personally I dont put much faith in it. I feel it has more to do with inbred lines than color . One thing with lavender though that I have seen while working in the longtails especially is, it has a feather type altering effect. It can at times make them brittle, some seem to stay in pin feather around the wing bow area all the time too. It's easily corrected by culling and breeding the ones that dont show this. But that is the only effect I have ever run into with the lavender color in general.
Now for the mille fleur question,
Yes it could be split for lavender, all depends. You say it has porcelain in it Heritage, that's the big question. Where in it's heritage? If it had a porcelain parent, then yes it no doubt is 100% split for lavender. If it was 2-3-4 generations back, it may or may not be carrying it (refer back to my breeding the color OUT post) On the F1 cross, all will be split
if you take these again back to mille, it cuts the ratio of splits in half, then again and again with each generation. AND all this assumes one of the parents was always a split, if it wasnt, then there is no longer any chance of lavender being in the line.
On lavender F1 splits, it's easy... They are ALL splits. From there on those you will be getting both split and non split black phase birds. There is no way to VISUALLY tell these apart. You have to breed them and see if they throw lavenders when bred to another lavender. If so they were splits. I never bother with splits outside of the first generation due to this, you end up test breeding them a year to find out what it was.
Now as to what it will produce ( your mille that is) all depends on what you are breeding it to?
Bred to milles = all milles
bred to porcelain (if a true split) = 50% porcelain and 50% milles ( all these milles will again be split lavender.
If you plan to breed it to another color, let me know and I will be glad to post the possible results.