Hi Dennis, hope your not getting upset? I did note a change in your post as it seems we are challenging you. I am not, I am just discussing. The point of discussing is to see if I have missed anything on the main drain topic.

In your previous post you say "Honestly, most of this is about actual experience and what you have to deal with".
It's the same for me. I started off in swimming pools just doing what I was told, believing everything the man in charge said.
My background is engineering, hydraulics and pneumatics so I do know a little more than the average pool builder. For me the greatest influence and most astounding turnaround was 4 years ago from the report on main drains and pools without main drains documents and links that mas985 posted. That was something that stuck in my mind and I thought over it for a long time. It's not rhetoric and the flow from the computer fluid dynamics is something used on aircraft design, space craft design, in fact anywhere where a gas or liquid has to flow.

I have played with pools similar to yours and noted the effects of opening and closing various parts and the quite frankly stupid ideas that the industry has as facts that we all follow. The key difference is I also have used the version without main drains and over large pumps and I couldn't believe the results. My pool is now a bit of a wreck as it is my test bed. I am fortunate that some of my customers have become friends and if I ask them can I experiment a little (no harm will come to their pools) they indulge me to play.

That main drain in the video, it's flowing at a massive 795 gallons per minute, 795! yet unless the dye gets within 2"-3 " of the grill it doesn't get sucked in it flows right on past. That fact is probably the single most important point on main drains.

The CPO teaches that 90% of the contaminants are in the surface of the water so that's where we should maximise the removal of dirt.

Now in your first post you describe the situation standard and the situation as you have modified it. Your sensible ideas have improved the situation (note I do not say common sense as I doubt what is "common" these days)

Water is incompressible so if you have a skimmer it should be matched with the same size in square inches of returns so 1 return for a skimmer is not rational, of course it will still work but the flow restriction from 1 return limits by definition the flow into the skimmer. I note you have 2 but you also have a big pump with a potential on high speed to still cause a restriction even with the M/D open because what come out the returns is what governs what flows into the pump. What kind of flow rate have you got?
I don't go with small openings in returns to speed up the water flow at the exit as this is simply physics, and for every action there is an equal and opposite reaction so the harder and faster you push the water in the pool is trying to stop it and the restriction in the return opening is having a bigger effect up stream with a slow down in water flow and more strain and pressure on the filter gauge.

If you do use the main drain to pull water for heating (what heater have you got that requires high speed?) and you send this out of the returns it still doesn't got to the bottom because it hot and less dense, it will float, sending the hot water into the pool from the M/D as a return does warm the bottom of the pool, does mix quicker and can cause any dirt to be lifted up into the mix to be skimmed out, you cannot get stuck onto the returning M/D and as you have stated this better flow with a main drain position (only returning the water will aid the skimmer performance and not reduce it. There should always be more than one skimmer although plenty of pools with only one do work, they work better with multiple skimmers and that can also handle the leaf load.