Not everything is known about the way color works, that's for certain.
As for dominant white, I must have gotten confused with another thing I read. Could have sworn something said 11. It is quite possible that there are other strains of Sabino out there (someone with more knowledge on color please help me here - have there been blatantly obvious Sabino horses that tested negative for the one Sabino test we have?).
Now, as a completely hypothetical example as I do not know if this has occurred, if a pinto-colored horse was blatantly Sabino no questions asked and tested negative for a current Sabino test, that would mean that there are other mutations of the Sabino gene. Tests for genes are created by discovering one might exist and eventually isolating it. So far one Sabino gene has been isolated, and if there are obvious Sabinos coming back with negative tests, that only mean there are genes that must be searched for and isolated (which is no easy task).
Any white on the face (star, blaze, stripe) is, in our assuming discussion, caused by Frame, Sabino, or Splash as Tobiano tries to keep white away from the face.
As for the birthmark theory you have, if you really think about it, what are the odds that so many horses would have these white markings in the same areas if they were just birthmarks? Why wouldn't they have white birthmarks on other areas of their body besides their legs and face? Even if they're not the same exactly, hundreds of thousands of animals having the same kind of birthmarks in the same areas is a little strange. I'm not saying it's impossible, but rather improbable.
Saying that no two facial/leg markings are exactly the same to make a sort of argument against them being caused by white pattern genes is moot. My reasoning for that being that no two louder pinto colored horses (obviously frame or obviously tobiano) have the same exact markings.
Take a look at Barbaro compared to my boy Reno. Very
similar stars. Now, maybe it's just random coincidence that they have near-identical 'birthmarks' or maybe it's a genetic thing. I'm leaning more towards genetically related...maybe only because they share a grandsire, but the similarity is there.
At this point I have no idea what I'm trying to say as I've been working 15 hour shifts and am exhausted XD