Kate... without me reading back through the whole thread, what are you looking for an answer to? I'm sorry, I have been sick the last few months and not able to keep up with the board as much as I have in the past. There really shouldn't be much to research (breeding test wise) as the genes you are working with are very well established and have been so for decades. I am a little confused by your using the term "splash" though in connection with various colors, so please clarify if you mean something else like excess white (all forms of which are also pretty well established). Splash in ducks is used in a way similar to splash in chickens. It is the result of homozygous Blue on extended black (in the Mallard derived breeds of ducks). Blue in ducks is incompletely dominant, as it is in chickens, so for example when you breed two Blue in the extended Black series together, you will get 25% Splash/Silver.
Anyway, as far as the Lilac/Lavender, it is in the extended black series as well- black plus homozygous blue and sex-linked brown dilution. How the genes work and their dominance has been established for many decades and it is a pretty easy color to breed for. I get questions about the color fairly regularly and as I have told other people, it is a rather easy, straight forward color to breed for, the problem is getting the color to look decent. The reason being is that all of the dilution genes found in the mallard derived breeds tend to lighten the plumage (often quite dramatically). If you look at birds that are even just extended Black with double Blue dilution (Silver Cayugas, Silver/Splash Swedish, etc...), the Blue dilution alone can lighten a solid black duck to almost white. When you add the sex-linked brown dilution to the mix, the plumage can be lightened even further and you basically end up with a lot of ducks that are pretty much white (feather wise).
I have no doubt the fact that the color is so hard to get to look decent is the main reason you don't see more of it. I am assuming the site you mention must be the calculator Henk created (he is a member here, but seldom posts in the duck section as ducks are really not his thing). If you have a link, I would love to check it out (if it is some other site). If it is Henk's site, I am not sure if he used the same genotype for Lilac and for Lavender. I gave him a PDF with all the duck genotypes a couple years ago now and on that it had "Lilac" listed as the same genotype as Lavender (extended Black, homozygous Blue dilution, and sex-linked Brown dilution, E/E, Bl/Bl, d/d). I would imagine the calculator you are using has them as the same as well, but it might be better for everyone to use Lilac for a different genotype (which is probably how it was originally), like perhaps the heterozygous birds (good luck though telling them from the others that are true Lavender).
Anyway, I hope this helps! Good luck with your project! It seems like everyone I have known of that has worked on that color has just given up because only maybe 1 in 20 come out actually looking Lavender. I know I have seen some awesome Runners in Europe in the Lavender color and to the best of my knowledge they are using the same genotype so it must be possible (to get the color to look decent, not basically so diluted it is white). You will probably just have to cull, cull, cull to get the color to actually look consistently Lavender. The recipe though for how to get to the correct genotype is well established. You are only dealing with those couple genes and it is very well known that Blue is incompletely dominant and Brown is a sex-linked recessive.
For further clarification, extended Black is an allele of the wild Mallard pattern. It basically turns a duck solid Black other than when certain other genes, like those for areas of White are present. All of these colors you are working with are on that extended Black base color/pattern (E/E)....
Black = Extended Black (E/E)
Blue = Extended Black plus single Blue dilution (E/E, Bl/bl)
Silver/Splash = Extended Black plus double Blue dilution (E/E, Bl/Bl)
Chocolate = Extended Black plus sex-linked Brown dilution (E/E, d/d or d/-)
Lavender = Extended Black plus double Blue dilution and sex-linked Brown dilution (E/E, Bl/Bl, d/d)
Lilac = Extended Black plus single Blue dilution and sex-linked Brown dilution (E/E, Bl/bl, d/d) *My thoughts on Lilac anyway.
As you can see, the colors are all closely related being in the same series. The variations in these colors are just the result basically of the different combinations of only a few genes. When you understand how they work to create the colors, it is very easy to predict breeding results and mate birds of the different colors together. It is all very predicable and well established (these colors/genes anyway! LOL). Again, good luck and I hope this helps!
