That's awesome and yeah, i can see the spammy issues taking place.
Maybe there's a way of doing a check while a user is typing and if they mention a username that their friends with to go a head and apply the mention, otherwise display it as plain text.

I know the Boonex team is busy, but It would be cool to see the team develop a WYSIWYG editor custom to the platform.

As for the New Message thing.
If you get a notification of a new message and you click on it from the menu, it takes you to the chat, but you need to click on the user who sent the message before you can reply.
Whatever chat you end up on when you click the "New Message" notification, maybe have the text field display.

I was going to do a short screen record of it, but can't upload videos on here lol.