Yes, Google has a number of penalties that actively hurt or de-indexes websites with shady links.

Just focus on getting backlinks that would be useful to people actually visiting those websites. If a human found your link, would they respect you for it? That's the question you should ask when you build links.

You're probably right about the estimate. I haven't done any testing on it or the impact of redirects in general personally, but have seen some folks suggest as high as 99% but others much lower.

Slightly. Like any redirect, you lose some of the link value. However, Google has said that PageRank does flow through shortened URL's. It just might only be 99% of the rank instead of the full value.

To add, is the link in the first couple of paragraphs, the middle, or the end of a document? Or is it in the sidebar, footer, or top menu bar of a site? Common sense can tell you which of these have more impact.