Why status in peer still shows "a/n" while radio is in "Only-N" mode was also my question but is not cleared in this. A software bug?
only-n is not really a special hardware mode. It is the same as a/n, except it does not accept legacy clients. When station connects to AP it is not distinguishing between those two modes, therefore displays "a/n". This approach is the same as "only-g" mode (it is also never seen in registration table), that has been there pretty long and it seems is not causing any confusion.
Hmm, I have been reading several articles (white papers) from within the industry (including the "802.11n-primer" presented at MUM in Poland) and they all consider "HT" the same as "Greenfield" mode.
802.11n has whole paragraph describing what is "HT device" and ability to support greenfield preamble is just one (by the way optional) feature.