Hi John,
I'll write you more on this later but in brief (I need to finish building computer today):
1) I really try to get clients to send me physical samples. If they don't... well, that is pure "guess-work". But there are some general value that real architectural materials used by designers end up with, as they only use terms like "matte" (still at least 0.4 gloss), "honed" (for semi-gloss marble 0.7-0.8). As for grazing angle, I don't use this often, and the effect is actually so very subtle in reality that even currently the material simulates it rather well.
And clients often tell me what they want, i.e satin wood, or varnished wood. They would not forget something like this :- ). But if they don't, I go for the most common natural treatment. It's not bad idea to start with almost all materials around 0.6 gloss. Everything is rather shiny but gets toned down with surface imperfections from microbump.
2) Fabrics are indeed my nightmate and I long petition Corona Devs to finally implement "SHEEN" like in DisneyPRX. Because they really can't be simulated well with simple fallof or reflection. And yup, I do the same as you, mix of two. Because lot of times fabrics, the actual fibers are reflective, so it's not wrong to use Reflection on them, but they are also "hairy?" or how it's called,and then creates the subtle brighter halo at grazing angle.
So my fabric are often both reflective (0.1-0.4 gloss, but so it doesn't end up shiny, I use it together with strong bump/normal map to break this reflection) and also fallof in diffuse slot.
I experimented with anisotrophy but that really doesn't work well...to get true anisotrophy we need something like GeoPattern to create the natural multi-directional anisotrophy that fabrics exhibit.
I still hate them, fabrics are still the worst looking thing in CGI without custom BRDF. I hope we'll be able to use VrayScans just to get their BRDF from them and replace textures (it's possible there already). This way we could use material property from Velvet, and just use our bitmaps and colors. But for now... just what you are doing.
3) I follow it semi-strictly, i.e 90perc. I break the rules occasionally but not often. My blacks are indeed almost never lower than 50 sRGB, and if that looks too grey ? Then you're not using enough contrast in Framebuffer and post-production :- ). That's how blacks really look in reality.
But there are already some really black materials in worls, metals, coatings, super-fabrics...so it's not like some of them aren't 20 sRGB, just that 98perc. of regular materials arent.
Glass: Yup, we often fake glass. In the image of close you mention, the reflection multiplier is like 0.1... I consider it to simulate "polarization filter" :- ). If you want to do this physically correct, use RaySwitch Map or MTL. Have the actual GI be pure reflective, but visibility smaller. That is exactly what polarization filter would do. You would still get correct light bounce, refraction,etc.. but you will see more into the glass. Smart fakes are absolute necessity.
4) HAH, the bed. Everyone asks and here is the secret: We never once forced clients, ALL OF THEM ask for those beds. 5 +/- years, or when it was, we created the first messy bed just for fun. And since than, clients still ask for that bed, they literally put it into brief to use that bed (or other messy bed they have).
I am pretty sure some people would not like it, and it would be poor fit for hotel room, but many clients are far less conservative than we think.