The ball valves need to go above the true unions. Think of it like this. Should you ever need to disconnect and remove the sump from the tank you will want to cut off the water from the tank and then disconnect the plumbing. This means you neeed the ball valve above the untion so when you disconnect the union the water from the tank doesnt go all over the place.
The concept of the union on the pump is so you can disconnect the pump from the plumbing with out cutting it out or doing to much damage to your plumbing setup. This is where a true untion after the pump is good as well as a ball valve above it so you can turn off the water going back to the tank and disconnect the pump.
A one way 'flap' valve can also limit the amount of water that returns when power is off. DONT get the ones that have metal springs in them. As you know metal and staltwater dont mix well. Marinedepot is one of the stores online that can sell you a metal free oneway 'flap' valve.
I second the suggestion for a larger return pump. Your going to end up with a head height of at least 4' or 5' and remember pumps dont have a consistant output at any headheight. The higher you pump your water up the lower the overall GPH
output of the pump.
You could connect the tub and the refuge by drilling a hole in the tub an dinstalling a bulkhead and plumbing. Rase the tub so the outlet thats maybe 3/4th up the side of the sump spills into the left chamber of your refuge. Dont run your fuge at full capacity as you will need to allow for water to flow into the sump and fuge from the tank when power is cut.