I hope somebody can explain how they use the words.
According to my dictionary there is no (or very small) difference between them.
A question about #1
What do you prefer in it: some, somebody, someone or pehaps anybody (anyone)?
I would say efficient is a way to describe achieving maximum productivity/ getting the most out of something. A good synonym for this could be productive or well-organised.
Whereas effective describes being successful or achieving an intended result. A synonym for this could be potent, successful or functional.
Great clarification Leia! Couldn’t have said it better