Im don't believe Neil had the diary the entire time. If he did, he wouldve given it to the MC way sooner than 19. I don't believe anyone died recently either, I think someone who knew Lynette went to the post office and sent the package to the last adress he or she knew about, hence Neil could accept it for the MC, because it was just your every day mail package.
Now lets go back to 19 and not related to MC's birthday either, since his dad told him about the package days if not a week before his 19th birthday, thats an odd date to send someone a package of that importance, right ? So why now ? That question drives me nuts ! Does the person, who had the diary and the dress in her or his possession, know someone at B&R, the MCs name got dropped, he or she guilt triped and sent the package ? Or did Neil met someone connected to his past, where he doesnt even know the connection and that triggered the package ?
Because "a week before he gets 19" its not exactly smth you put into a will, so it seems impulsive and reactive, like smth mustve happened that trigged it, but what ? Oh, how I sometimes hate DPC and his mysteries.