[Lsufan]

If u want to go with a c2c box then u can use the lower hole as the emergency drain, so it will have a elbow facing up. It doesn't look to be but about 1" below the other hole, so if u use it as the emergency u probably won't even have to make the box any deeper then normal to accommodate it.
I don't think I would use it for a return because u would have to rely on locline to not have a bunch of water drain down when it's powered off. If the locline gets pushed down for some reason it may cause u to have a flood. If u have a really big sump with a lot of extra room then it may not matter.

U can eighther go a full c2c & use it as the emergency & go over the back with the return or u can go with the internal wier & exterior box & use it as one of the pass through holes. I know uncle isn't a fan of that setup & he is more knowledgeable then I am, but I like the synergy style boxes. I just don't like the sizes they make them in. I like to have as long of a wier as possible on the inside of the tank & a bigger exterior box then what they make. One of the things I like about this setup other then the space I save is u can use black acrylic for the internal box & have the bulkheads hold it on the tank, Then do a glass exterior box. I really like the look of the black acrylic. I don't know if it would be worth the extra work just to save 1.5" in the tank, but I wanted a acrylic box so between the two it was worth it to me.

As far as the returns, if u don't want to go over the back of the tank then from the looks of it u have enough room between the edge of the tank & the existing holes to drill another hole for your returns. If u did it this way then u could have a longer internal box. U can make the internal box to where it covers your existing holes & still have room for returns on the ends as long as u have around 6" from the edge of the tank to where your existing hole starts.