What's the difference between 왔어요 and 왔었어요?

ANSWER: one means "came" and the other means "came (and went)" 

This morning I picked up the book pictured below and read the article also pictured below. The article talks about the difference between 왔어요 and 왔었어요, and gives the following two example sentences to help show the difference:

1. "아까 친구가 왔어요." 
2. "아까 친구가 왔었어요." 

The article says the first sentence means "A friend came a little while ago (and is still here or maybe not)," and the second sentence means "A friend came a little while ago, (but is no longer here). The article says the first sentence is in the past tense and the second is in the past perfect tense. 

But a man named Lee Su-yeol (이수열) claims that "past perfect" is alien to Korea and has been adopted into Korean from English. Instead of saying, "아까 친구가 왔었어요," Mr. Lee claims Koreans would traditionally say, "아까 친구가 왔다 갔어요," which translates as, "A friend came and left a little while ago." 

Even the KBS research team, the people who wrote the book, explained that "아까 친구가 왔었어요" means "아까 친구가 왔다가 돌아갔다," which translates as "A friend came and left a little while ago." In other words, the KBS team used a sentence that Koreans clearly understand to explain a sentence many Koreans apparently do not understand. 

So, if you disagree with Mr. Lee's claim that the past perfect tense is alien to Korea, then why is the KBS book explaining 왔었어요 instead of 왔다 갔어요?

Finally, which sentence is correct and why? 

1. "우리 전에 본 적이 있었죠." 
2. "우리 전에 본 적이 있죠." 

Sentence (2) is correct because "우리 전에 본 적이 있다" translates as "We've met before," which means the sentence is already in the past tense and, therefore, does not need the past tense tag "었죠." 

The first sentence seems to be imitating the English-style tag question: "We've met before, haven't we?" But Koreans just say, "We've met before, right?" as in the second sentence. 

So, it seems English grammar is influencing Korean grammar.