Hi Yair. I am a Hongkonger. I think I can help you.
I can say 喺度 and 喺處, but can I say 喺便?
We say 喺度 indicating 'here', but we seldom say 喺處(恕) now, it is an old way to say it.
However, for 喺便, 喺邊 is a better pronunciation for it. 喺邊 is not used in a statement like 喺度 and 喺處 do, instead, it can only be used in a question, meaning 'where'. Because 邊度 means 'where', the exact location is unknown.
Question: 你喺邊（度）？ (Where are you?)
what is the difference between 頭先 and 啱啱 (meaning: “just”, as in “I just saw my friend in the street)？
頭先 and 啱啱 have the same meaning.
頭先我向戲院見到Mary 同David! and 啱啱我向戲院見到Mary 同David!
mean the same.