One thing missing in all of these discussions. Actual hatchery steelhead witnessed on the spawning grounds with wild fish.
As said previously, the issue isn't even necessarily hatchery spawning with wild, it's hatchery fish spawning period. Even two hatchery spawning together would create a "wild" offspring. If any of those offspring come back, hatchery genetics are then spread to the wild population. There's a good amount of evidence that hatchery genes, even after just 1 rearing cycle in the hatchery, are passed from parents to offspring.
As for how many hatchery steelhead are actually spawning, it would be incredibly hard to know. You can't really swim during the peak spawn due to visibility and height. You can't radio tag every hatchery fish in the river just to have 80% of those killed and hopefully returned (the cost is huge and won't happen). I can tell you I got 2 hatchery kelts last spring in April in the lower and I only fished under 10 hours total, so they are obviously spawning somewhere.
At the end of the day, creel observations are being used, and they will assume that up to a certain % of hatchery fish spawn because of that 58% release rate. It's the best they can feasibly do. As much as we'd like to know, conditions in the Vedder in May don't allow for accurate spawning observations.