I think I may have found an error in the schema, according to the manual.

In the manual the guidance is provided that for a document that an 
element either the milestone form or the container form must be used 
exclusively in a document. And to use both in a document is an error.

 From the manual:[...]

If this is truly the rule, then <div> cannot not be milestoneable 
according to the schema today.

The element <osisText> does not accept mixed content and only allows 
<header>, <titlePage> and <div> elements. The <div>
element cannot 
contain <header> or <titlePage>, but can contain other <div>
elements, 
many of other elements and also text. If <div> is used in its milestoned 
form, then the there are no child elements other than other milestoned 
<div>s that the <osisText> would allow. This would lead to a
document 
that is not very useful, having only milestoned <div>s as the "content" 
for the <osisText> element.

Or
If there are two element <A> and <B> such that <A> contains
<B>, and <B> 
is milestoned, then the only elements that can be between the milestone 
begin and end for <B> are children that <A> allows.

The intention of milestoneable elements should be merely to handle 
boundary crossing elements. If that is the case, then the elements 
between a milestone begin and end of B should be only those element that 
are allowed as children of B. But the schema rules define parent child 
relationships and in this case the elements between the a milestone 
begin and end of B are actually children of A. Thus the rules for A 
apply. This results in that what is allowed and what should be allowed 
may be partially disjoint sets and only the intersection is useful.