We need a possibility to notify the user first. So parenting Direct Messaging

Wouldn't it be a notification? Not a direct message?

Yes but I would use the DM system to let users befriend eachother

Three possibilities:

1. Will be send to the server as @{id of user}, so when a username changes the references don't get lost. Server will then check if the user with the id is a friend of the author. Client will then replace the id with the full username.
2. Will be send to the server as @"{username}" (and the server replaces that with the user id before it writes the post into the db), server checks again if they are friend. Client will have no way to link to @Users profile.
3. Will be send to server as @"{username}" (saves to db as @{id of user}) and the server replaces it with @"{username}" <{id of user}> (similar to the From header in emails)

I would choose eighter 1 or 3.