You don't need 6 inches between the two drains - you really only need enough to keep the bottom drain submerged (not sucking air) while the top drain is still JUST above the water level. It's probably more like 2 inches in most setups (Assuming the siphon drain is a downward facing opening, i.e. an elbow pointing down). The extra height is nice because it gives you more wiggle room when adjusting the siphon valve but it is not a requirement.
The emergency should always be at least as big as the siphon - if the emergency is smaller, it might not be able to handle the full flow if the siphon gets plugged.
If the siphon drain is pointed downward, wouldn't that increase the risk of getting air in the upside down U part? Could I have the siphon intake pointed 90 degrees to the side?
I am tempted to put in a 1.5in for the return. It seems like it would fit in a 6x4x5 inch internal overflow box. Are there any unforeseen space issues I would run into? this is the box I was looking at but it seems all the places I look at use the same dimensions. http://www.bulkreefsupply.com/sumps-...rflow-box.html