1. unless you have a big thin content problem there is no gain
2. completely reversible, just remove and wait
3. you will have to decide, you seem like you are on the right track.
4. Question you should have asked, is there any downside to no-indexing these pages, Answer Yes there is, all links pointing to a no-indexed page will leak all their link juice, noindex is a last resort, I have never used.
if you must noindex a page, do it with a meta no-index,follow tag, note that was "follow", not "no-follow", then your link juice will flow into the page and back out again.