Of course, the use of the word "may" leaves the door open to direct regulation by EASA, I fully accept that.
You clearly don't accept that Article 7 of Part-FCL is exactly that "direct regulation". Let me have one more go.
Art 7 paragraph 6 of the Basic Regulation says:
6. The measures designed to amend non-essential elements of
this Article by supplementing it, shall be adopted in accordance
with the regulatory procedure with scrutiny referred to in
Article 65(4). Those measures shall specify in particular:
(a) the different ratings for pilots' licences and the medical
certificates adequate for the different types of activities
performed;
(b) the conditions for issuing, maintaining, amending, limiting,
suspending or revoking licences, ratings for licences,
medical certificates, approvals and certificates referred to
in paragraphs 2, 3, 4 and 5, and the conditions under
which such certificates and approvals need not be
requested;
(c) the privileges and responsibilities of the holders of licences,
ratings for licences, medical certificates, approvals and
certificates referred to in paragraphs 2, 3, 4 and 5;
(d) the conditions for the conversion of existing national pilots'
licences and of national flight engineers' licences into pilots'
licences as well as the conditions for the conversion of
national medical certificates into commonly recognised
medical certificates;
(e) without prejudice to the provisions of bilateral agreements
concluded in accordance with Article 12, the conditions for
the acceptance of licences from third countries;
(f) how pilots of aircraft referred to in points (a)(ii), (d) and (f)
of Annex II, when used for commercial air transportation,
comply with the relevant essential requirements of
Annex III.
In other words, EASA is to make implementing rules to flesh out the details of the previous paragraphs of Article 7.
Part-FCL is the collection of those "measures", the implementing rules. (a), (b), (c) and (f) are covered by Part-FCL Article 3 and Annex I. (d) is covered by Part-FCL Article 4 and Annex II. And (e) is covered by Part-FCL Article 7 and Annex III, which is entitled "REQUIREMENTS FOR THE ACCEPTANCE OF LICENCES ISSUED BY OR ON BEHALF OF THIRD COUNTRIES".
Thus Part-FCL Article 7 and Annex III describe the mechanism by which "the requirements of the second and third subparagraphs may be satisfied by the acceptance of licences and medical certificates issued by or on behalf of a third country as far as pilots involved in the operation of aircraft referred to in Article 4(1)(c) are concerned."
And when Part-FCL Article 7 says:
Third country licences
1. Without prejudice to Article 1, Member States may accept third country licences, including any associated ratings, certificates, authorisations and/or qualifications and medical certificates issued by or on behalf of third countries, in accordance with the provisions of Annex III to this Regulation.
it doesn't mean
"Member States may accept third country licences ... in accordance with the provisions of Annex III to this Regulation or alternatively by just accepting the licences issued by third countries without further ado."
It means that Annex III describes, exhaustively, the ways in which Member States are permitted to "accept" third country licences.