"Is the windage knob protected enough to keep it from accidentally being moved? "
I don't know that remains to be seen. But, you need to put a witness mark on the windage knob so you know if it has been moved and if so, you can return it to zero.
"For a dedicated irons-only rifle, I'd prefer an A1 receiver.
I don't think it makes any difference to me. My rifles are not dedicated iron sight rifles. But, if my optic goes down, my BUIS now becomes my primary sighting system. Now we both know that I will never be involved in a long term SHTF senario, but, I want to know that if my optic goes down, I can use my iron sights as my primary sighting system indefinitely and not sacrifice much in the process.
When I took this carbine class, I used a Troy BUIS with a single aperature: the large aperature. I found it very difficult to maintain accuracy at any kind of significant range. I had taken this course twice before: once with an Aimpoint ML2 and once with a TA31F ACOG. With the BUIS, I shot the worst score I have ever fired and struggled to do that. I had to really pay attention to the fundamentals like cheek weld with that large aperature. At 200 yards I was horrible. I believe (or maybe I want to believe) that this wasn't all my poor skills with iron sights. I shoot some matches fairly frequently with iron sights and am usually competitive.
With this GG&G sight, IMO it gives me the advantage of being able to use either aperature with both having the same zero. It is also a very low profile sight which is important when the sight is not deployed. I originally went with the Troy sight because my ARMs #40 sight was too tall when stowed to allow me to mount an ACOG. So, I replaced the ARMs #40 with the Troy.
"The GG&G MAD (Multiple Aperture Device) BUIS (Back Up Iron Sight) was originally developed to fulfill a request from Naval Surface Warfare for a back up iron sight that provided both a large and small aperture on the same plane and would thus be zeroed to the same point of impact. "