This is a large range of subjects to address completely in one post. Can I shotgun a few quick opinions?
I think your goal is to have detritus settle in the sump/refuge/filter, and to have plant growth in the filter dominate plant growth in the main tank. I think one parameter that needs to be met for this to work is rapid turnover between the display tank and the vegetative filter. Another parameter that needs to be met is that the plants in the filter are in an optimized environment for uptake. They should not be limited by gas exchange or light. The last parameter is that the filter be reasonably easy to maintain and harvest. As far as how big the filter should be compared to the display, IME 20% is big enough in my setup. As far as what plants to use, maintaining a variety of easily manageable macroalgae is effective, IME. Inland Aquatic's flora kit, which is a variety of alga Inland has found easy to cultivate, would be a good start IMO.
Rapid turnover: it sounds like you're addressing this in your design. 17% of my display volume passes through my filter, per minute.
Gas exchange: in a typical tank/sump arrangement, most of this will occur due to a surface skimming overflow and an air-drafting overflow tube (like a Durso). Passive or active air flow across tank surfaces and near the overflow can help.
Light: I use an EYE Color Ace 6500K on my 65G display for its first year. I then use the bulb for another year on my 12G vegetative filter. What I'm trying to emphasize is the filter is illuminated much more intensely than the display tank. Along with a lot of water flow of well-aerated water, the filter is a more optimal environment for vegetative uptake than the display.
Maintainable: I don't like rock in a vegetative filter. IME it restricts water flow through the algae and make removing settled detritus difficult. Also, alga like Derbesia are easy to scrape from plastic tank walls, but difficult to harvest from rock. Same thing for soft-tubed fan worms, which do a great job of binding detritus in their tube walls. I also don't like sand substrate in a vegetative filter. This is a personal preference. I keep 5" of sand in the display, none in the filter. The water coming out the OF pipe has lots of bubbles in it. Bursting bubbles can salt over a light fixture in a few hours if you don't do something to contain the spray.
My way is one way to do it, and I make absolutely no claims that it is the best. I don't believe "natural" systems are "best" for maintaining all types of stony coral, they are just one way to accomplish the goal. For dedicated SPS tanks featuring Acropora, I admire some of the tanks with no soft substrate or vegetative filtration at all (Ex: Bomber, 64Ivy) and think that these tanks can be well-suited for maintaining these type corals.
