Regarding your companion, the booking class will be that used for award tickets for non-ROP Gold (O for first, I for biz). Further, just be aware you can't use any tools like checkmytrip.com to view award bookings since they remain internal to the THAI system.

Recently, I paid for discounted biz ticket (D class), used my GUP2 to upgrade to first (in P), and used more of my miles to get my kids biz award seats. All worked out fine, but finding availability for them was hard than for me. THAI staff did a great job to help me work things out (final routing BKK-LHR//FRA-BKK)