The heat has to go somewhere. If it’s going into the river that’s very bad ecologically. They don’t address this in the article though.
Not necessarily. I put heat into rivers every time I swim and it's a total non-issue.
Heat could potentially be an issue in sufficient quantities - if for example Google doesn't have sufficient dissipation infrastructure before the water is returned to the river. We don't know whether or not this is the case. It would be irresponsible to make that assumption with a complete absence of evidence.
It's actually a bit of a problem now that the power plants are starting to shut down.