The original Standard of Perfection, (which was lifted almost word for word from the UK Shetland standard) was, I think, intended merely to ensure a sound animal with a good conformation, and to allow the "type" to be interpreted as wished.
Even though there are many "types" of Arab, the end result is still an animal that can be discerned , nine times out of ten, to be an Arab, not a carthorse.
This is not so with Minis, there is no set type, within the standard.
Most Walkers would be easily identified as such, even if they differed form the set norm slightly, you would still be able to see they were Walking Horses.
The very fact that people are successfully breeding part Arab Pintos, and that there are registries and shows for these horses, shows that there
is a set idea of what an Arab looks like.
The only set idea for a Mini is that it is under 38 or 34 inches high.
Nothing else.
Most people do not care what it looks like, so long as it is small, hence the active market for dwarfs.
The only place that you get any idea of where the "breed " is heading, type wise, is in the show ring and there, just as soon as we think "Yes,
that's what a Mini should look like" it changes.
Bit like the weather
You cannot breed a Mini to another breed and get a "half Mini" it does not work.
Until the type is set, until you can look at something that has gone over height and see that it is an over height Mini, even if the books close, it will not, automatically, be a breed.