With my medical hat on (Cave Rescue doctor for ~20 years)
Suspension syndrome/trauma and crush injury are 2 very different conditions. They are often taught together, which is not helpful, as while they may appear similar, the underlying pathology and treatment is quite different
Suspension trauma is principally thought to be due to the pooling of blood in the legs in a vertically suspended non-moving causalty. Return of the blood to the heart requires muscle activity in the legs to compress the veins, so if there is no muscle activity, the blood will not flow back. Eventually, this will lead to an inability of the heart to deliver enough blood to the rest of the body. When that starts affecting the brain, the casualty will start to pass out/faint, and the issue with being suspended is that you then can't go to a horizontal position. If you remain vertical, the inadequate flow of blood to the brain will start causing severe harm, and potentially be lethal. Getting the casualty into a horizontal postion should help equilbrate the pressure in the blood vessels, allowing blood flow to return to the brain and other vital organs. In surgery, we often cut blood flow off to a leg (e.g. knee replacements) or the whole lower body (operations on the aorta) for up to 90 mins, before reperfusing them. The time for oxygen starvation to start causing irreversible brain damage is much less, so the worry about toxins building up in the legs is not really relevant here - by the time that may have happened, they will almost certainly be dead from oxygen starvation to the brain. Using a tourniquet will not help, but has the potential to cause severe harm, such as loss of a limb.
In addition, it will probably be hard to get specific post-mortem findings if someone dies of suspension trauma, especially as the reason they are immobile is probably also contributory (hypothermia, injury etc). Experimentally, people have started getting symptomatic after only a few minutes, but there is certainly variability between people, although the exact reasoning isn't clear. Either way, the key point is that passive suspension is potentially dangerous, and someone incapacitated on a rope should be got down as soon as possible, or positioned horizontally if they can't be got down. Once down they should be positioned flat/recovery position if unconscious. The W position (knees up, sat up) has no evidence to support it, and is not recommended. As for a conscious casualty - if they have remained conscious throughout then they have not had full suspension syndrome, but potential pre-syncope (near fainting). It would be prudent to have a period of time recovering before mobilising again, but the majority of the guidance is written for people with full suspension trauma; i.e. unconscious. The HSE document remains a good resource (I accessed it at
https://www.hse.gov.uk/Research/rrpdf/rr708.pdf)
Crush injury results from direct muscle damage from high or persistant pressure applied to cells, resulting in cell death and the release of cell breakdown products. There is a possible role for tourniquets, but more to control any catastrophic bleeding on releasing the pressure. For example, if your femur has been shattered by a boulder, it is quite possible bits of bone have ripped one of the major arteries, which is then compressed. When the boulder is removed, the artery is no longer compressed and it might get a bit bleedy very quickly. Tourniquets can be very useful to stop catastrophic bleeding, but the role in preventing backflow of toxins into the body is significantly less clear and they are not currently recommended for this.
My medical advice would be to avoid both of these conditions