>>15
You didn't answer
>>14 at all. You talked about ている when the other person wanted to know about てくる. Well, specifically he wanted to know about 行ってくる. Rather, in that case, it is 行って来る because the idea is "going and then coming back."
行って来る is functionally equivalent to the English phrase "I'm heading to XXXX. I'll be right back/be back later."
For example, when you leave the house in the morning to go to work/school, you'll say out loud 行ってきます. You're saying, in essence, "I'm leaving, but I'll be back later."
It's just a fixed expression, though. No one thinks of it as "i'm going and coming back." Instead, it is just thought of as the equivalent of "see you later". I mean, no one actually thinks about the actual meaning of each word in "see you later" when they say it, do they? No! They just say it as an idiom. 行って来る is an idiom as well.