Hi, Being a new member myself and only having experience with ameraucanas (and other chickens) for a few years now, I certainly would not consider myself an expert, but I will take a stab at answering your questions.
The second question is fairly easy to answer. 'Blue' in chickens does not, in fact, breed true, it is the result of a black bird having one blue diluting gene (I'm not sure what the technical terms of the genes are), while two diluting genes result in what's known as a 'splash'. When breeding black, blue, and splash. different pairings give you different results,
Black+Black=100%Black
Black+Blue=50%Black and 50%Blue
Black+Splash=100%Blue
Blue+Blue=25%Black, 50%Blue, and 25%Splash
Blue+Splash=50%Blue and 50%Splash
Splash+Splash=100%Splash (With the possibility of a 'Ghost Splash', a bird that's genetically splash but is lacking in 'splashes' and is nearly white in appearance).
Lavender on the other hand, is caused by a different, recessive diluting gene so it needs two copies to express. What is commonly thought of as 'lavender' is usually a dilution of black, so when breeding a blue with a lavender, the result is the same as breeding blue to black, giving you blue and black offspring, however these will also all be carrying one copy of the recessive lavender gene (It is also generally not recommended to mix blue and lavender as subsequent generations can result in 'lavender blues' and 'lavender splashes' and there is already enough confusion between blues and lavenders because of the 'self blue' naming issue).
The first question is a little more difficult. It is certainly true that roosters tend to be significantly larger and somewhat stockier than their female counterparts, but a healthy hen should still be well fleshed out. That being said, on a healthy bird the keel bone is usually felt fairly easily, but it should not be extremely prominent. If she really seems abnormally thin, there could certainly be something wrong, either stress, illness, parasites, or a combination of these. Try treating her for worms and other parasites, and I would also suggest maybe separating her and feeding a her a diet higher in protein and fat to see if she starts putting on weight.
That's all I can really think of at the moment. I hope it helps. Maybe some more experienced members will have more advice to add.