Greetings is prefered. It is the same way with thanks. When you say 'thanks' to someone, you are implying that you are thankful in multiple ways and that you were helpful in multiple ways. You don't want to have to describe each of the ways, but you want it to be known that the appreciation is significant and you don't want your expression to be limited in measure and scope. The same feelings are involved when you say 'greetings'.

"Greetings from XX" is best. Alternatively, you could say "This is a greeting from XX", but still the first choice sounds more natural.

😊 greetings would be used here

Greetings plural is correct. In fact this holds true for any phrase of this type with a countable noun. For example: Cars from BMW Songs from the Beatles

I think "Greetings" is best, Yvonne. By the way, if you're looking for a tutor, this is my ID: 8722548 Hope you have an awesome day!