a) You mean discount business, not discount economy
b) adds up for me, comparing against the earn tables (https://www.qantas.com/au/en/frequent-flyer/earn-points/airline-earning-tables/qantas-and-jetstar-earning-tables.html)
- Discount business 18600 + 280SC - matches your bronze figure
- Flexible economy (for WP status bonus) - 12400
- Total 18600 + 12400 = 31000 (plus the 280SC) - matches your platinum figure
https://www.qantas.com/au/en/frequent-flyer/status-and-clubs/status/platinum.html