jsherloc - 5:42 am on Apr 12, 2012 (gmt 0)
1. Google has no clue who builds the links and they state they don't care in their guidelines/reconsideration process, they just have an issue with certain types/velocity used and want those issues addressed as much as possible. Fact is, most people will not be able to remove the majority of "unnatural" links from their overall site portfolio, as we've already acknowledged most websites out there pick up tons of "shady" links along the way through no fault of the owner.
2. Just in my own experiences the past year helping clients out, we've seen specific examples of spun blog posts, article directory posts, forum profile links, blog rolls and others shown directly to our clients directly from Google. Google shows you some of these types of links as an example of what they are having an issue with.
Just about every single response I've seen directly from Google showing these types of "unnatural" links they have an issue with, shows types of links that can certainly be pointed to your competitors, typically en masse, and very easily/affordable at that.
3. Not sure exactly what you want here, as backlink graphs and logs of onsite/offsite changes across all types of verticals and competition-levels are a start, and about all that those of us on the other side of the curtain can compare/contrast experiences with. What "proof" could we demonstrate to you in this regard? The volume of webmasters noticing these similar issues and sharing their experiences within their industries leads me to believe something strange is afoot.
Correlation is not causation, but when enough people report similar things and you see it repeatedly over and over testing and isolating the variables as much as possible...it is clear that there is an issue of some sort.
edit: The site(s) mentioned in the Google thread(s) aren't mine, but as I've mentioned, I've been following this issue very closely the past year as I believe it is only getting worse out there, so it was interesting to finally see SOME type of public response from a real live Google employee