Hi @radjr,
You should be able to apply the patch to 3.3.0-14, but we haven’t tested that and can’t provide support for it. A much better plan would be to upgrade to the latest 3.3.0-x release – you’ll get other fixes too.
I don’t see {$Username} anywhere in the codebase; I think it must’ve been added manually or something.
The available email variables will depend on what template you’re working with. You can check what tags are available by composing that email, then using the 
tool in the compose box to get a list. This gets much more complete in OJS/OMP/OPS 3.4.0 and later, when we rewrote the email tools.
Regards,
Alec Smecher
Public Knowledge Project Team