Hmmmm. I think you're right. The easiest (without draining and re-doing you system) is a hang on overflow. I've heard nasty rumours about them and they appear to have a greater risk of flooding for some reason. I've never used or seen one so not sure.
I'm wondering if there is some way either replace the 4" PVC sleeve with one that has overflow teeth and somehow get it to seal where it meets the bottom of the tank to create a centre style overflow. Then you could just cut that one drain so it's 3"-4" below the water line inside that centre overflow. I think you said you have a gate valve, which you would put on that line to control the flow....
I'm just not entirely sure how you could create that seal between the bottom of that 4" PVC and the tank...
As far as the skimmer tub is concerned, if you didn't want to or can't get a full sump yet, try looking for a deeper tub. One that is the same height as you sump. You may also want to either increase the size of the bulkhead joining the 2 or add another one to allow quicker flow for when power fails on you get a sudden rush of water back into the sump system.
That's all I got
Maybe someone else can chime in about creating a centre style overflow in there without tearing it all apart..