I wonder if Greenpeace is doing more to undermine confidence in all academic research, undermining confidence in climate change science much more than the denial of it.

This was my first thought. The only difference between me and the climate change skeptics I know is that they have no faith in the integrity of scientists, and I do. "See, the oil and gas industry has been buying off scientists!" won't help my argument, they'll just think that it's a countervailing force to all the scientists on the other side of the question who are being bought off by …whoever they think it is.

Edit to add: Furthermore, I don't think this is unreasonable of them! Figuring out which experts to trust is a really hard problem in a society as complex as ours has become, and this makes it harder to argue for "you should trust scientists".

I assume this happens already. It's unlikely that with so much money at stake, only one party is doing the dirty. Same applies to any issue where a lot of money is at stake.

Who could gain by doing something like that?

Anyone who stands to gain from carbon legislation - be they traders, taxing entities, or "remediation" consultants