While I agree with your final conclusion on the effect of bilateral lower extremity amputations on overall hemodynamics in elevated G-force situations, I disagree with the path you took to get there.
Blood pressure is not dependent on "the amount of blood in the body and the amount of resistance provided by veins and arteries".
Blood pressure is equal to cardiac output times peripheral vascular resistance (which, by the way, is almost entirely provided by portions of the arteries referred to as arterioles). Physiologists USED to believe that total blood volume was the main contributor to "preload": the pressure present in the ventricles (the chambers of the heart that pump) when they have filled up and are about to squeeze (end-diastolic volume), but they have come to realize after extensive studies on distributive (septic and neurogenic) shock that this isn't always the case, and almost certainly wouldn't be the case in high-G situations. See, under normal physiological conditions, venous blood is returned to the heart under a normal venous compliance (increase in pressure that occurs with an increase in volume), but under abnormal physiological conditions (I think 8 G's qualifies as "abnormal"), the veins are not able to generate sufficient pressure to return blood to the heart, preload is not increased, and the heart cannot increase stroke volume in order to increase cardiac output. This is why, when someone is in septic shock, you can't always give them enough fluid (or more blood) to maintain their blood pressure, so you have to give them medications that increase the resistance of their blood vessels in order to maintain perfusion pressure to their critical organs (read: brain and heart) until the primary offense has been treated.
The reason your ultimate conclusion is correct, though is not physiological. It's anatomical: VALVES.
The deeps veins in the extremities (meaning the big veins), including the legs, have valves that prevent backflow. This means that blood CANNOT flow from the rest of the body through the venous system, and the arterial system is already a high pressure system, so G forces don't really affect it that much. Thus, amputating the legs wouldn't really make a difference, because the venous pooling that already occurs in the abdominal organs (the real reason venous return decreases with G force) would still be present, because the body has already accounted for backflow to the legs through evolution.
The reason G suits MAY work is that they increase venous return from the legs, regardless of blood volume. Theoretically, they would increase effective circulating volume because they increase the amount of blood available. But remember, during high-G situations, blood ACTUALLY pools in the abdomen, not the legs. And this is the reason they MAY not work. But we don't know, because it hasn't actually been empirically studied. Someone came up with a theory based on our understanding of hemodynamics 30 years ago, someone made a product, and now we do it because reasons. So the reason they're used is what medical researchers often refer to as "voodoo". This is something that seems logical from a physiological standpoint, but has never been exhaustively tested, but has become standard, so now no one is willing to challenge it. This is the case for G-suits. Total voodoo.
Good show, though.
