1. If AD skips all of training camp this year he'll push out his free agency by a year. He has to report with about a month before the season starts in order to accrue a service year this year (and keep on tract to reach free agency next year). SO he'll be in no later than early August ... which, based on last season, will give him enough time to get ready for the season opener.
2. The Rams can extend Donald this year without affecting the 2018 cap. See my other post in this thread ('No, that's not true') for details.
AlbaNY_Ram