I won't add anthing about the queen excluder, its' functions and success or lack of success in honey production, but as for an upper entrance, I have what for me is the ideal solution.
When a good flow is going on and crowding is a real concern, I make a temporary upper entrance by sliding the super above the excluder or above the brood nest, back about 1 to 1 1/2 cm. This produces an entrance slit all the way across the front of the hive. The entrance is from the front of the first super and not the back (because if the different thicknesses of the wood) and does not open the back of the hive. As soon as conditions no longer show the extra entrance to be needed, slide the super back to its normal positioning and, "Walla", the extra entrance is gone. Quickly enough the field bees once again get used to entering from the regular hive entrance.