Ok, I think this is how you should do it:
One wire to a fuse block. Don't worry about the distribution block then several fuse blocks. The individual devices should be fused, and I really couldn't tellyou the ratings for each, without knowing the size. The radar detector should fused no more than 1 amp. The USB hub should be about 2.5 amps.
To turn the USB hub on with the OPUS, you could just put a relay on the power line from the battery, and use the accessory line from the OPUS to turn on the relay. Pretty simple, eh?
To turn on the inverter, I wired a relay right to the power terminal on the inverter. Then a switch running to the relay. I don't think you will be able to put it into the compartment. I have it in a large enclosure in the trunk, and I've had a few problems with overheating. An external fan fixed it pretty quick though.