This is a common misconception. How does crouching down give up his height advantage? I would argue that it does the complete opposite as a taller boxer like he was. By lowering himself to his usually shorter opponent's eye level, Foster's jab reaches further. If you think about it, punching in a downward trajectory shortens the punch by a few inches as opposed to punching straight forward. Foster fought in a very sided-up stance which gave his punches even more reach. Foster's sided-up stance and the way he concaved his body also gave him more protection. You are essentially minimizing the naturally bigger target that you are as a taller fighter if you fight like Foster. He saw punches coming from a mile away in that stance as his head was literally further away. Punching downwards also opens your head up more and makes it harder to keep your chin down. I would argue that by crouching down, Foster can target the opponent's body more safely, especially with straight punches, and perhaps get more opportunities for counters. Foster knew this as he often targeted his opponent's stomach and chest areas with his excellent jab. Of course, the drawback of crouching is that it restricts movement. When Foster had to be more elusive or wasn't as aggressive, he usually rose to a more neutral standard boxing stance. Finally, by crouching down Foster takes away his shorter opponents' natural advantages. The shorter guy has more leverage as he punches from the lower position. By crouching down, Foster takes away that leverage and gives himself more of that same leverage. Short guys are also used to ducking and weaving under the punches of the taller guy, so by crouching down the short guy feels more uncomfortable, especially when at the same time they have to deal with a humungous reach like Foster's. I would say that as a taller boxer, Foster is one of the guys that you would want to mimic as he truly made the most of his tall, lanky body.
I disagree with his part of your post. Your shoulder is lower than your head and your reach is maximised when your jab is at a 90-degree right angle relative to your body. If a 6ft 3ins fighter stands up right and sticks his arm out at a right angle from his shoulder, thereby maximising his reach, his fist is square in the face of a 6ft 0ins (roughly the average height of a Foster opponent at LHW) opponent. Crouch to roughly your opponents height and you have to angle your arm up slightly to jab them square in the face, thereby not quite maximising the full extent of your reach. Additionally, your shorter opponent doesn't have to angle his jab up to quite the same extent as he would were you standing 3 inches taller than him, either. I've no problem with any of the other points you make in your post, with regard to the benefits Bob's crouch gave him and haven't quoted them.
Foster is one of my favourite LHWs of all time, but I do think he's a tad overrated due to being so dominant over mediocre opposition and overall impressive on film. I don't rank him nearly as highly as others do, though I do think highly of his H2H ability.