I am not an expert by any means, but here it goes...
The agriculture tubs/troughs sound like a great idea. I would ensure that they are washed out and cleaned though. Humidity shouldn't be a HUGE problem as long as you make some sort of cover for the tubs. There are several ways you can tackle humidity problems. Thick plexy glass would work best as it is easier to cut, and possibly less expensive than glass. Also, will cut back on topping off. If you still have humidity problems you can put a bathroom fan/vent leading outside, or utilize a de-humidifier. I think the vent would work best.
As far as The closed loop v.s Open, I am not as familar with. As far as I know a closed loop uses a drain to run water to a pump, back to the aquarium for maximum water flow, open would include a sump with a return pump. ( correct me if I am wrong). It sounds like you will be using an open loop system with your 2 sumps, but inorder to create more flow ( with out ugly power heads) you can also do a closed loop system on the back of the tank on top of the return pumps from the sump. Now that may seem to make things complicated, while you could just drain and pump more water from the sumps, BUT the way I see it, if you allow low flow in the sump, it will act as a place for waste to settle, and make it easy to siphon out, while more flow will keep it suspended throughout the tank. Just ensure the turn over is more than adequate.
Other things to consider-
- Will the sump room have a drain? ( in case of over flow)
- Are you looking to run the Frag tank off of the Display?
- Natural lighting?Best lighting out there. I've heard of very good growth, and no cost after install ( especially this size) You can alway include blues/acintics for "looks".