Planet13 - 11:13 pm on Dec 24, 2013 (gmt 0)
Hi there, Dan:
I would also like to echo aakk9999's question specifically about the time BEFORE your drop - i.e., the significant RISE in traffic during March 2012 that preceded your drop.
That does seem to be characteristic of a pre-penalty phase.
Was your rise in traffic prior to the drop a seasonal occurrence?
Getting back to your questions:
"1) What is the sure proof way to make sure a link is 100% bad?"
I don't think there is any 100% threshold of proof, and I would be worried if anyone claimed there is one.
If you listen to John Mueller and Matt Cutts, they never seem to speak in absolutes about good and bad links. They often say things like, "If you have links that look spammy, you might consider adding them to the disavow file."
Look at how they mentioned to deal with Press Release links and guest blogging. They said that they are links that you might want to have nofollowed.
(I apologize in advance if either of them have come out and been more definitive about exactly which links will lead to either manual or algorithmic penalties, or if either has given a more definitive "threshold" of links that would cause problems.)
So long story short, you really want to remove anything that looks obviously spammy.
However, as mentioned above, Penguin recoveries seem incredibly few and far between.