I don't use fleece rollers, so maybe I'm wrong, but shouldn't the water from the overflow flow through the fleece roller for it to be effective? Assuming the fleece roller is working correctly, you can definitely ditch the floating floss. As mentioned, just floating the floss is ineffective, and even if it was effective in theory the fleece roller and the floss are occupying the same role, so I think one or the other. You don't need to overdue mechanical filtration in my opinion, a lot of successful reefers including Josh Earle from World Wide Corals don't run mechanical at all.
And, just floating the bag of carbon will definitely do a little, but it's not nearly as effective as having the water flow through the bag of carbon. I see you have a 3-way Tee and valve on your return line, consider buying or DIY'ing a simple media reactor which could be fed with a hose from that Tee, and run your carbon in that.
I hope this helps!