To answer your questions on the BB waist strap, I would say that it is more about stabilization and comfort but has some good ability to distribute the load (I believe a framed backpack would be best for that).
As an example of the waist strap carrying the load - when wearing the waist belt snuggly, I can loosen the shoulder straps and let most of the weight be held via the waist strap and it feels comfortable – essentially the shoulder straps become the stabilizer and the waist strap the load support.
Now, I have a caveat that may make my experience slightly different than for someone using a new BB:
I have a 2004 version of the Brain Bag with a 2” non-removable waist strap – not the 1.5” version that is removable today.
However, I feel, based on the 1.5” waist strap I use on my Smart Alec (yes – also an older version with a bit larger waist strap than today’s verison), it should still do the job fine on a BB.
As for the longest I have had a BB on my back, I would say 4 hours with a medium load (2 large 1+litre thermoes, many blankets, heavy laptop + accessories (power supply, Bluetooth speaker, etc). However, I have used my BB on airplane trips where I one-bagged it (except for laptop) with heavier loads – carrying it on and off for 1-2 hours at a time and many times (waiting for plane, getting off plane to desiination, etc) knowing that I could go a lot longer.
For those times, I really did not feel anything was on my back and sometimes would forget I had it on (my wife had to remind me to take off my backpack when I was just standing at the window watching the airplanes land and take off). The BB was very helpful to my back.
Have a look at this forum post which talks a lot about the waist strap and its use.
Backpack question: shoulder straps + waist straps
As far as managing all the straps, I use the Velcro strips like the Strapeez product TB sell. I use it to manage the compression straps and the sternum / waist straps See this link for good ideas as well (I also posted on another way I deal with the shoulder strap ends):
Corralling (Sp?) Backpack Straps
I like using my BB as an EDC as well – especially when I may need maximum expansion options. It compresses well. Your selection of accessories is great. The key to packing this bag is to pack vertically and not allow things to piled at the bottom of the bag. The Freudian slip is great for this. The Lead’s Pocket is great (it is padded too) as it can hook into the Annex Clips on top. You can even fit a Co-Pilot below it in case you want a mini-bag or small camera bag – the Packing Cube Shoulder Bag (PCSB) works great too as well as the snake charmer.
I cannot help you with color but the Navy and French Blue are good conservative colors if that is the environment you are working in. Steel is good too.
I hope others can chime in here on their thoughts for any heavier loads they have carried or other experiences with the waist strap as well as their general use if similar to yours. I am sure there are a lot of other suggestions for color and accessories that work with the BB as well
Let me know if I missed answering any of your questions.
Hope this helps