You could use a separate prosecutor for these cases, maybe even appoint a defence attorney as a special prosecutor.

Isn't this the exact point of Internal Affairs departments?

The point of the internal affairs department is often to appease voters by looking like they're doing something about police corruption. They're incentivized to go after high-profile career making cases, not the run-of-the-mill "cop says one thing and the videotape says another".

Making it easy and profitable to take clear-cut cases like this to civil court adds incentives to go after these sorts of things.

As I understand it, Internal Affairs departments tend to be investigatory rather than prosecutorial authorities.