D-efficiency is the metric for design quality. Orthogonality is one component of efficiency, but efficiency is what we want to maximize. So I'm not aware that there's a better test. Looking at efficiency and at one- and two-way level balance the way you're doing really is a great way to evaluate designs.
You noticed that medium price is being chosen more often than low price. When you run your utilities, do you see that medium price has a higher utility than low price? As long as your imbalance isn't too bad, the multivariate analysis should account for the fact that medium price and promotion are shown together more often and in apportioning utility it should separate the promotion and medium price utilities. I would run utilities and see if this is happening.