I do agree with you that a few more pictures and video on the TB site would help. The narrative descriptions are good if you can take a few moments to wade through them, but it's not the same as seeing the bag from all angles.

I don't have the Brain Bag. I have a TriStar, Western Flyer and Synapse. The Synapse is a backpack but would likely be too small for your needs and the other two are travel bags, but I have packing cubes for the TS and the WF which are, in each case, specifically designed for the bag they go into and fit perfectly. Also some of the inserts are available in a variety of sizes to suit different sized laptops.

At the risk of sending you into research overdrive, you could post a query on the TB forum over at their site. There are legions of TB groupies there who will be able to share real life experience, with photos, with the Brain Bag (there is bound to be a thread already). Lots of them seem to use it as their every day bag, in a very similar way to what you have in mind. But I am not sure if it has a sleeve to sit it on top of a rolling bag. But given you plan to return all but one of your review bags anyway....