Hi! Your question is very complicated but I will try to answer it as best as I can.

1. First of all, "look alike" this phrase means you resemble another person. You could either say:
"You and your sister look alike" OR "I think my mom and I look alike" You are comparing two or more things to each other that hold a resemblence.

2. When you say "alike" it means holds similarity. For example "This green apple and that red apple taste alike" It means they have a similar or same taste. Now if someone IS a "look alike" it means they look the same or similar. For example, some people tell me my celebrity look alike is Jennifer Anniston or Avril Lavigne. So "look alike" could either be a phrase comparing, OR a phrase used to tell someone who the person is that looks like them. Does that help?

3. The next part with "looks as if" and "looks as though" is harder. It depends on the tense...


Now (present): "It lookS as though it will rain." OR "She lookS as if she has seen a ghost" they both mean pretty much the same thing.

Or: "You look as if you've seen a ghost." It depends on the phrase

Past (already happened): "It looked as though it WAS going to rain." OR "She looked as if she HAD seen a ghost" 

Past: looked as if/ looked as though
Present: looks as if/looks as though 

People don't use this phrase "looks as if or looks as though" that often, so don't stress about it too much. Its more common to just say "It looks like it will rain" OR "You look like you are going to be sick" with using the phrase "look/looks/looked like" depeneding on the tense the word is being used in.
Hopes this helps!