• Bonsoir pour au revoir lorsque l’on se quitte en fin l’après midi ou juste avant la nuit.
• Bonsoir peut se dire aussi lorsque l’on arrive dans une assemblée aux mêmes horaires, plutôt qu’un tardif bonjour.
• Bonne soirée sous-entend : passez une bonne soirée (en général lorsque quelqu’un a l’intention de passer la soirée en dehors de son domicile) si l’on quitte quelqu’un, ou que ce dernier s’en va.

Précision :

• Bonsoir s’écrit en un seul mot lorsqu’il s’agit d’une locution.
• Bon soir désigne une fin de journée particulière :
  Vous allez au théâtre ? Que ce soit un bon soir pour vous !

That’s not a question, in fact, but many.

First of all, that’s Bonsoir in one word, not Bon soir. But indeed, the general principle you quote at start is mainly correct (meeting/taking leave of someone).

For the confusion you had with people responding with Bonsoir when you left, although it’s probably rarer than Bonne soirée, it’s possible also. The only “too odd” usage would be Bonne soirée when meeting someone you’re going to spend the evening with. Saying Bonsoir when leaving is acceptable, though more formal than the other alternatives you quoted (Ciao, Salut).

When meeting someone on the way back home (but no actual conversation intended beyond simple greetings), use Bonsoir any time, but Bonne soirée if (and only if) you can suppose your interlocutor hasn’t actually finished his soirée (in other words, if you somehow know he’s going home to sleep, in case you should use Bonne nuit). And, in a way, it makes sense: it would be pretty absurd to wish someone an event in the past. Following this rule, you “can” (and should ! friendliness is a nice thing…) certainly say Bonne soirée to someone on his way back home, if he just mentioned he’s having a party at home for his birthday for example. And of course, you cannot say it if he just said Je n’en peux plus, je rentre me coucher.

In other words, using Bonne soirée (or Bonne journée) has in fact more to do with your interlocutor’s activity than it has to do with time or sunlight. Use a neutral Bonsoir when you don’t know.

For your last question, Merci, bonne journée, au revoir sounds (to me) redundant (symbolically if not technically) and is slightly too “heavy”. Merci et au revoir ! or Merci, bonne journée ! sound best if I may.)

bonjour, bonsoir are greetings
bonne journée, bonne soirée mean have a nice day, have a nice evening.

A native French speaker would never say:
“Vous allez au théâtre? Que ce soit un bon soir pour vous!”
Instead you could say: “Vous allez au théâtre? Passez UNE BONNE SOIREE!

reference: pages 223 to 227 Le Français déchiffré, clé du langage et des langues
Henri Adamcweski

“Bonsoir” is standard and can be used when you arrive somewhere (in the evening) and when you leave (in the evening).

On the other hand “Bonne soirée” is a little bit more involved and can be used as a goodbye wish when you leave. I have always felt that there is something unpleasant and intrusive in the “Bonne soirée” wish. Why on earth do you need to wish a good evening to somebody when you leave : it seems to mean “Now that I am leaving, I sincerely wish that the remaining part of the evening will be nicer for you”. It carries some elements of bitterness and dissatisfaction.