I did not run any signature campaign but this is just my observation yes it will improve your SEO during the duration of the campaign if the campaign stop then back links will be taken down, but the official thread of the signature campaign will continue to give you backlinks, on your third question it depends if the forum is a Do Follow sites to know more about Do Follow you can go here
What Dofollow Backlinks are and How Your Business Can Use Them on your question number 4 and 5 it also depends of the page rank of the forum where you held your signature campaign and if the forum is a dofollow.
couple of people replied here with a yes answer but this works mostly if the site is pleasing and clean from negative reviews because you cant get viewers if new people find this out . not all can run s cmpaign for along time because this is taxing but having a campaign with a shorter time would still create some awareness than nothing at all than those who did not try . the reason why theres also a long running signature camapign is because they dont want the traffic die but rest of you have said do helps not just the validity of the campaign .