In my mind, "bonan tagon" means the same than "bonan matenon", so, you can say "bonan tagon" in the morning when you want to greet somebody, or say "bonan matenon" as well
I believe you can also use the verb form of "komencant*" and say "Mi komencantas".
Funny. Looks like words from different languages mixed together... French/latin, german, english, spanish/portuguese. Bonne/bonam Tag, me estoy/estou/estás comenzando/comencando...
Yes, that's exactly how Zamenhof created Esperanto. He took bits and pieces from different European languages. Not just the Romance and Germanic, but the Slavic as well.
No, it's not. Correct me if I'm wrong, but I feel like it should be, though.
Good day often is a way of saying goodbye in english. I'm assuming this is a greeting though.
Yes, some Romance languages literally say "good day" where we would say "hello". For example, in French they say "bonjour".