Darude should only "disappear" after you build the storage in Central for the trade events with Orchel. But it requires to finish the quest with Lucilla in order to get to Orchel, so he shouldn't be gone before you do the quest. He should still be in front of the castle in Aldlyn for the quest wih Lucilla.
Can you please send me the save file per mail, so that I can check what is going wrong for you? Since someone else reported the same issue, I assume there is a bug that was added with the recent change to the trade route system.
Please send the save file with the issue to Naughty-Netherpunch@web.de
You can find the saves in the \www\save folder of the game. I only need one .rpgsave file with the save slot number in which you have the issue. (For example, it would be file1.rpgsave if you have saved in the first save slot.)
Sorry for your troubles.