"You again, Calum!"
Translation:Thusa a-rithist a Chaluim!
I would say that if you're using someone's first name, that's the definition of not being on formal terms with them, so sibhse shouldn't be used. If you're on formal terms with someone, you would use their title, such as Doctor, Professor, etc. or you'd be addressing them as mother, father, grandfather, etc.
That's my view based on my overall knowledge of languages where the formal and informal singular exist, not based on a deep understanding of Gaelic grammar.
That seems reasonable, but I note you say "sibh" shouldn't be used in those circumstances, not mustn't. "Sibh" is not wrong therefore. In the UK it is increasingly common for strangers to introduce themselves by their first name only. I imagine other places do this too, to make selling easier, for example. English does not have a T/V distinction, but France and Germany do. So I suspect they would still use the formal You even after giving their first name, and I (for one) would not be "thouing" them.