"I'm searching for information on English-speaking countries' manners and customs, ... a baby's birth (or weeding, visiting hospital, going to funeral)?"
This is an easy one. Tell the florist what occasion it's for. For any happy occasion or for a hospital visit, any colorful assortment is fine.
By the way, in the U.S. the people giving the wedding usually provide flowers for the wedding party's outfits and for decoration; guests give presents but don't bring flowers.
"How to accept or refuse invitation? What's the origin of ... such introductions, so hope to get some help here. Thanks."
"Try any of the books written by Judith Martin, often known as 'Miss Manners'. She is spot on with her advice, and humorous at the same time."
Seconded, but in view of Steve Hayes's comment, I'll add that she wrote about U.S. manners.
Jerry Friedman