score:5
Yes, you can check in early. The confusing wording is attempting to say that early check-in opens 12 hours before the flight, so checking in and leaving your bags 7 hours before departure is perfectly fine.
Note that early check-in takes place at a dedicated "Early Check-In Lounge" behind Terminal 1, Row 5, a short distance from the regular BA check-in counters in Row 8, and this lounge lets you check in early for most oneworld airlines (and some others) in Changi.