ANA counters open 3 hours before TOD (it's listed for Bangkok here http://www.ana.co.jp/wws/us/e/asw_co...nter/open.html).

Yep, THAI's lounges are utilized for ANA.

5:15-5:30 sounds like a very reasonable time to arrive at the airport. There is essentially no one going through security/immigration at that time of the day except for ANA pax. Expect to spend 15 minutes total with check-in and security/immigration.