In response to #1, unfortunately the answer is its impossible to know. We've had campaigns where disavowing links brought near immediate results, but we've also seen some that took months to correct.
As for #3, its more of a judgement call than a hard line on DA. As with all SEO, if there is good quality content on the page, then its a fine link to keep. If it looks borderline, probably better off disavowing it (better safe than sorry).
