Karen, you make a good point. I suspect that there will be a system default for which authority it gets its labels from. In our case, we're using the ID one. Whether that's expressed in the data would be important when sharing the data, but not necessarily internally.
On the issue of alternate labels, implemententations will have to decide how much of the real authority to access and cache locally, depending on their system's capabilities. At any point, the bf:Authority could be updated by linking out to that authority and pulling back changes or more full info. Or it could subscribe to a push service from the authority.
Alternate names that are only used locally will need to be supported as well in the bf:Authority, since a local archive may have names that aren't in the authority "file".
As far as "where is the link for updates", I think it would probably not be the same link, since so many flavors of update would need to be handled. A system would need to know how to interpret the link and get to the flavor of update it wants (JSON serialization of the full info, rdfxml of just the label, etc). Each authority would have to maintain an API for such updates. That said, the ID link, with content negotiation, is actually a good step in that direction, since you can get all these formats of the record right there:
We do not, of course, have a push service that tracks change dates (yet).
Here is the example of a BIBFRAME Authority from the Authorities document:
IFLA Study Group on the Functional Requirements for Bibliographic
<hasIDLink resource="http://id.loc.gov/authorities/names/nr98013265" />
<hasVIAFLink resource="http://viaf.org/viaf/148620313" />
<hasDNBLink resource="http://d-nb.info/gnd/2167628-8" />
I still haven't an answer from an earlier question (now lost in all of this email) as to whether there will be a specific link from the BIBFRAME Authority to the actual shared authority file used by the library - that is, the file from which the library derives what here is shown as the "<label>". The example above shows links to LCNA, DNB and VIAF, but it isn't clear if any one of those is singled out as the authority being used by the library. Why does this matter? It matters because if the library intends to be part of an authority community, they have to be able to receive updates from the shared authority file, and therefore there must be a link between the shared authority file and the local usage of a term. I illustrate this in the diagrams I did at: http://kcoyle.blogspot.com/2013/05/bibframe-authorities.html (see esp. 2nd diagram).
If there isn't such a link then I do not see how libraries will be able to keep their names in sync with the authority file to which they adhere.
We also haven't talked about alternate names. The examples show a single name form. Indexing requires alternates. Both single names and alternates often comes from a shared authority file (that isn't local) and both types of name forms can change. What is the link that makes change management work with the BIBFRAME Authorities?