oh hey this is where you guys all went!
All you need are two drains to run a herbie as already mentioned. One for the main and one for the emergency. It looks like you have a third in your overflow box? I'd either cap that off and don't use it, or 'T' two of them together as your main and use the third as an emergency. Another option is to use the third as a second emergency which I think you were planning but that is a little overkill. Nothing wrong with that though I guess. If your drains are 1.5" it will be very hard to overflow a wide open 1.5" drain unless your return pump is rated for something insane like 3000gph (don't forget about the loss of flow due to elbow, head and such), which it probably won't because then you're pushing way too much water through your sump in my opinion
Another thing, those holes that you are planning to use as returns, I personally would not use them. They are a bit low and will probably cause overflow in your sump (unless your sump is massive, like 200g and only run at half level) if the power goes out. You can throw a check valve on those but you will need to remove those check valves and clean them frequently otherwise they will gum up with junk and eventually fail. Or worse, what happens if a snail gets in there? If it can happen imagine that it will :-) It may not happen today or tomorrow or next month, but that's the problem with Murphy right? You just don't know when.. Instead, I would run your return up and over the edge of the tank and while you're at it I would run the return pipe to the other end of the tank (opposite from the return overflow).
There's really no right on wrong answer really, whatever you end up doing, if you get water into the sump and out of the sump then it is working. It all boils down to how efficient you want to make it, how much maintenance you want down the road, how much of a failure alleviation factor you want to put in, etc.. Based on my experience with peninsulas that's what I would do :-) There are a crap load of them on canreef, just do a search and you'll find them :-)