Let me reorg the discussion:
1. Every single building used for boarding purposes should be an `aeroway=terminal` + `buidling=*`, because `aeroway=terminal` on a `buidling=*` describes the property of usage instead of strictly indicating the whole terminal, which is the practice in OSM. All the examples raised by you (Except FPO) are notated as `aeroway=terminal` + `buidling=*`, even the remote gates hall in LAX (https://www.openstreetmap.org/way/413226763) are notated as this.
2. From the examples you raised, I see the practice of using `type=multipolygon` + `aeroway=terminal` is quite common. Although I don't think it's a perfect solution, I accept this.
3. I insist that `aeroway=concourse` should not be used on `building=*` independently, it should only be used to describe the concourses under the same roof with other components. Further, there is no document or guide for this tag provided, so it is not understandable to any data parser. 
4. HKIA official website map not only puts all T1, NSC, and MFC in one tab, but if you drag the map, you can find they also put SkyPier and 4 car parks in the same tab. The map clearly separates main T1 and MFC as 2 independent parts, as you can find that under T1 it writes "(Gate 1-80, 511-530)" while under T1MFC it writes "(Gate 201-230)", where T1 does not cover MFC. Interestingly, In this map T1 does cover NSC, which makes me believe it's okay to mark T1 and NSC as one building, but that is based on your suggestion of "if Sky Bridge is considered as a `building:part=bridge`". This is not "conflicting with my preferred definition", since main T1, Sky Bridge, and NSC have already been marked as a single continuous building. 
5. I totally understand HKIA's current and future situation. HKIA is not the only airport using this structure. For example, LAX Midfield Satellite Concourse (https://www.openstreetmap.org/way/831597425) is a satellite concourse only of TBIT; ORD T1 Concourse 3 (https://www.openstreetmap.org/relation/13749364) is also a satellite concourse only of T1, they are mapped as separate terminal buildings, while there are also other terminals in the same airport. I believe it is good enough to follow the current practice.