9. ipv6InterfaceTable -
   ipv6InterfaceEffectiveMtu - like you said, can be easily computed.
   ipv6InterfaceReasmMaxSize - my experience is this is not per interface.
        
   "ipv6InterfaceAdminStatus: does it make sense to enable/disable
   IPv6 on its own on the interface?"

   One will want to enable/disable a given Address Family on a 
   interface shared by multiple address families. 
   I've seen implemented systems where there are entries 
   in the ifTable for IP Interfaces that provide the functionality. 
   I'd prefer a single table to enable/disable an 
   IP Interface(struct ifnet) as opposed to a port. 
    
   See: http://www.nanog.org/mtg-0102/ppt/mibs/sld051.htm
        
   "ipv6InterfaceOperStatus: other than the above, noIfIdentifier(3)
   is this one's only useful state, which can be determined from
   the Address table if DAD failed or there is no v6 address on
   this interface. [not efficiently, though]"
     
   I prefer we have an OperStatus if an admin status is included. 
   It would be useful even in IPv4 to see what else might fail such
   as dhcp assigned addresses or in a particular implementation
   some lack of resources, etc.
        
     ipv6InterfaceIdentifier        Ipv6AddressIfIdentifier, 
     ipv6InterfaceIdentifierLength  INTEGER,
     ipv6InterfacePhysicalAddress   PhysAddress
        
   I suggest the compliance section state it is acceptable 
   that MIN-ACCESS be read only.
   Most interfaces for  will compute these
   and even for interfaces which may not be straight forward
   like serial, the current drafts stil provide a way to
   have an implementation generate acceptable values.

     "When can this be different from the address of the
     interface's protocol sub-layer, and why?""
        
   I am aware of at least one IP router
   that ships with a cache of operator assignable 
   IEEE MAC addresses to provide to user define 
   IP interface.
   All physical ports in the system have one hardwired MAC, hence
   the ifPhysAddr of the port != ipNetToMediaPhysAddress
   of the given IP Address.