How can you change the welcome greeting? The choice is fullname or username. But some people have long names and then when you add the word 'welcome', this may take up a lot of horizontal space and will not look good on mobile. So how can I just show a person's first name without the word 'welcome'?
1. The language key 'MOD_SCLOGIN_WELCOME' defines the "Welcome" portion of the message. You can set that key to an empty string.
2. Joomla does not differentiate between first name and last name for the user's name. If you look at the com_users DB table, there's only one field called 'name'. To do what you're asking would require custom code to break apart that name. We don't do this in SCLogin. However, you can create a template override of the logout.php file and add PHP code to split the name by spaces yourself. However, this may be error prone dependent on the format of the name / language / etc.