I usually avoid these types of judgemental topics, but here goes...
I like to apply two rules for determining ethhical behaviour while fishing, and I have broken them- I'm no Saint.
1. Am I acting in a socially responsible manner- i.e. in compliance with the rules. If targeting steelhead then you were fine.
2. Am I actively demonstrating care for the environment. It sounds like you took extraordinary care to minimize the adverse impact of hooking coho. Sounds good to me- I believe they will still do their thing and spawn.

I just wish there wasn't the title that says- "target coho..". It seems inconsistent with your dialogue.
let your conscious be your guide.