What's the difference between "coucou" and "salut"?
Coucou is rather colloquial and used mostly for kids, very close friends and family members. You don't use coucou when you are introduced to someone, but salut would work.
Why not "Allo, je suis en retard ..."?
From what I understand, Allo is primarily for the phone. Not that you get notes which might have been helpful at this stage!
Can you also just say "j'arrive" to mean "I'll be there soon" or "I'll be right back"? I've seen it used that way by native speakers.
I thought Coucou was a person's name, such as Coucou the clown. One of Duo's little tricks!
I thought it was a term of endearment between like man and wife.
What is wrong about tout à l'heure