Well, a cooperative relationship of course would work best. Unfortunately the historical circumstances that gave rise to unions didn't include such a relationship. Unions became favorable because of abusive practices by organizations (employers) with an incredibly lopsided advantage in resources (financially and politically). That hasn't changed in the last 100 years.

I agree completely. Countries with better employer-union relationships are typically countries where workers have more legal rights. So outlawing abusive practices to create a better balance of power would help.

Defending a member in a trial/adversarial process and a cooperative relationship do not exclude each other. The union can very well defend the member in this case while still discussing cooperative solutions to the general problem of officers bullying citizens, just as a lawyer can speak for your side while still reaching out to the other side to find a solution that's acceptable for both sides and does not include a verdict.

It is however important that unions have a bargaining power that is at least comparable to the other side's power - and that sometime requires going head to head with the opponent.