I have thoughts and feelings about a lot of this, but the part that stands out to me is LE folks intentionally working with agents out of their jurisdiction to circumvent the laws in their own jurisdiction.

You want to talk about unethical behaviour? That sounds borderline like a poison tree to me.

Follow the incentives.

The only practical check acting against the whims of these agencies is that if they do things that are too horrible the resulting public perception will be bad for the career advancement prospects of the top ranks who want to move into politics where optics matters.

Isn't that like half the raison d'être for the five eyes?