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Where do I start... I love a good challenge.
Any cross with any of your Lavenders will show no lavender in the F1 cross. It is recessive. It produces something we call "split Lav". You have to cross the F1's to get any Lavs. 25% will be Lavender.
Who did you get your Lavenders from? Lavenders are usually solid black birds with a diluting gene that makes them lavender. There are at least two lines of Black Ameraucana's out there that Lavenders could have been bred from, and I know that some of them are Birchen based (sound familiar?), and some Extended Black. I just read that on the Ameraucana forum a couple nights ago. It makes a difference to some degree what their background genetics are. It's possible some Lav birds also carry Blue.
It also seems that most of the Blue and Splash Marans in the U.S. are also Birchen based, meaning BCM or Silver Birchen Marans with genes that cover the usual coloring of Birchens.
When you say Blue Splash, I assume they are Splash, homozygous blues.
So:
Blue Splash Marans x Lav AM Roo: Blue or Blue Birchen split for Lavender - carrying only one Lavender gene that will not display. You need two to see the Lavender effect.
Blue Splash Marans x Silver Am Roo: Most likely something that looks like a Blue Birchen, blue with silver hackle and saddle, possibly some salmon color on hen's breast.
Blue Splash Marans x Blue Wheaten AM Roo: You will get the normal split of 25% Blue and 75% Splash, but likely with lots of gold bleeding through from the Wheaten gold.
BCM hen x Lav AM Roo: Black birds with a little gold or copper bleeding through, split for Lavender.
BCM hen x Silver AM Roo = Gold Birchen roosters, silver birchen hens, with varying amounts of hackle color, and possibly color bleeding through in breast of hens.
BCM hen x Blue Wheaton AM roo--this is the only one I can guess at: 50% blue and 50% black, but what will the "wheaton" do? Most likely create BCM looking hybrids that are incorrectly colored, with copper that is too light, and black that is insufficiently dark.
With what you have, the most interesting cross would be the BCM x Lav AM. If your roo has a good beard, then all your F1 birds should be bearded, peacombed, and feathershanked.
These are all dominant traits. None will be Lavender in the first generation.
Your hens will likely be all black. Roosters may show some gold hackle, saddle and wingbow color. If you want to retain any color other than the lavender then select your F1's with the most color in the hackle. If you want to retain the olive egg, feathered shanks and bearding, select your F1's with the best peacomb, feathering and beards. Cross these selected F1's and you will have your Lavender OE's. One in Four will be Lav, of these Lavenders, one in four will have a straight comb and may not lay an olive colored egg.
You can cross your F1's back to the Lav AM roo, and get a larger percentage of lavenders, but you will lose some hackle color and some feathering on the shanks.
From what I have seen, the hackle color in lavs from these crosses don't look that great. It just looks dirty yellow. To avoid this, select your F1's with the least hackle color.
Have fun!
So I don't have much luck with BCM Roos I have Cuckoo Roo no problem nice and healthy. 
