I think all of the above is important too, Bet.
But is the head domed in sm cavaliers because thats how the dog is or because they have pressure in the skull making it that way ?
I think maybe my simplification of a complex subject has led to some misunderstandings.
Cavaliers do NOT have domed heads -- King Charles spaniels by contrast have a very domed head. Some cavaliers have a more rounded or domed skull than others, though, which would be expected in a breed developed from KC spaniels. Some breeders who have viewed many, many MRIs of affected dogs believethe space for the brain is MORE truncated in domed skulls because the occiput (back of skull) is shorter. The space where the brain is compressed in SM cavaliers is NOT at the top of the head (eg the dome of a domed skull)but at the base near the occiput. Most brachycephalic breeds have some compression of the brain in this area as a 'norm' (though that's not saying it should be an 'acceptable norm').
A flatter skull is actually the normal and natural shape for a dog's head, as you can see by looking at, say, a labrador or springer spaniel. Some breeders were bred to have a domed skull zand this type of skull is sometimes associated with problems with hydrocephalus (water on the brain) as well as a failure of the skull plates to close completely (considered 'normal' again in chihuhuas but usually considered a potential risk for breeds as there is literally an opening directly to the brain).
Researchers all say they have seen no correlation to an increase in SM in dogs with different shaped skulls but likewise I do not think there has been a comparison of more domed compared to flatter skulls. Breeder Laura Lang, who is quite well versed on anatomy, reading MRIs and health issues, believes she sees a distinct difference, to the extent that she is no breeding for less rounded, roomier skulls. My Jspoar, who os clear of malformation/clkear for SM, has a broader, flatter, skull. Unfortunately this is not a headshape considered desireable right now. That said it may merely be coincidence that he has this head shape and is clear/clear. Far more research is needed before any conclusions can be drawn on such issues but my gut feeling is that Laura is probably right to be breeding for the head she is interested in and that she feels is healthier for the breed.
To me it appears there are many factors going into the shape of the cavalier skull and also many factors that seem to affect how the CSF flows and gets blocked in SM cavaliers. Researchers have done some preliminary MRIs with a machine that can view fluid flow over time (called cine-MRIs) and I think doing more work in this area (as well as many others of course!) will help us all to understand better what all these relationships are.