1. This is almost exclusive to sustained free hangs. Bouncing against a wall or otherwise working your thigh muscles circulates blood with minimal effort, so standard climbing circumstances raise the safe time - potentially by hours. (Notably, Alpine climbing guides and specifically Alpine rescue guides do cover this syndrome in case someone falls and hangs while awaiting aid.)
2. OSHA-style harnesses are over-the-shoulder devices anchored between the shoulderblades. Many don't even have waistbands, just leg and armpit straps. That's useful for unpredictable falls, but it's the worst possible configuration for suspension trauma. Your legs hang straight down and the straps pull up at the edge of your groin, cutting off blood flow. Even hanging free, the climbing harness posture (elevated legs, pressure on hamstrings) is much safer.
3. Safety harnesses are mostly cheap and ill-fitting because you're not expected to hang in them; they're the same unpadded straps you get on the worst climbing gym rental harnesses. The time to initial risk apparently varies a lot with harness fit and padding, so comfy climbing gear is also much safer.
The ad below has a good visualization of the difference; it's from a company selling OSHA-compatible harnesses which pull you into a climber-like sitting position for exactly this reason.
https://www.rocorescue.com/ALL%20BLOG%20IMAGES/FreedomHarnes...