The idea that a second wash is some big deal for scalp health seems way overblown to me. Most people's scalps are fine and the first wash already lifts the oil and dirt, so that second round is mostly just stripping whatever is left and making the client feel pampered. Where I could see it mattering is heavy product buildup or someone who goes a week between washes, but for a regular client coming in every 4 to 6 weeks, does it really change anything? Color taking well has more to do with the prep and how you apply it than whether you lathered twice in the bowl. People act like this one step is the line between healthy hair and disaster, and that just does not match what I have seen. Test it on your regulars like you are doing, but I bet the differences you notice are small and mostly in how the hair feels right after, not in any lasting scalp change.

Seen way worse on big box jobs honestly, half those crews are subs getting paid by the fixture so speed beats clean work every time. Crooked straps bug me too but a missing drip loop is the real issue there.