If I was faced with your decision, I would probably apply for the Commission. The command/IPT tours would come later, so you would have time to enjoy the 1st and 2nd line work, and grow into your role as an Engineering Officer. Of course, OASC could reject you for many reasons, but the fact you applied will work for you, and may help the subsequent AT application. And as an AT you can re-apply for EngO at a future date if you still wish to pursue it.
Just my take.