Quite a complicated setup.
I think you don't have to use the canistor. It's a nitrate producer.
From main tank, straight away go to your sump with filter wool.
Put skimmer inlet after the filter and output near the return pump.
UV.... can put the same method as the skimmer.
I think you need a denitrator for your fish and invert tank as you will tend to overfeed the fishes.
You can put the GAC just before the return pump.