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What an awesome episode!! There were many questions answered, but of course, even more were raised. Let's start with what we now know:
Eloise is in fact Dan's mom, I know I was one of the doubters of that being true. She also is the young Ellie we met on the Island in 1954. What I never saw coming is that Widmore is his dad! That was a major surprise, but it makes me wonder where he got the name "Faraday" from...
Dan did some kind of experimentation on himself for his research before he tried anything on Theresa, which evidently caused him to have a memory disorder (think Momento but not as extreme). As Widmore promised, the Island seems to have cured him.
Widmore was the one who planted the fake Oceanic 815 plane at the bottom of the ocean. Do you think his reason for doing that was just so he could make sure Dan got to go to the Island?
Desmond is going to be ok, whew! :)
Dan's further research has led him to believe that you can, in fact, change the past, which could result in changing the present/future. Evidently "humans" are the variable he hadn't considered before, so he thinks that if he can detonate the buried hydrogen bomb (to destroy the immense magnetic energy of the Island), this will negate anything that happened from that point on, including the 815 crash. We haven't seen anything to prove he's right, though, and we know that Eloise knew she was going to shoot her son back in time from what she said to Widmore about sacrifice. She could have easily told Dan not to go to the Others w/ a gun, therefore her past self would have had no reason to shoot him, so why didn't she? And why did he insist on talking to Eloise in the first place?
So now what? Are Jack and Kate going to try and do what Dan wanted and blow up the bomb? Neither one of them looked that excited about the idea of erasing the past. What would happen to them and the other Losties if it worked?
Other random questions, how is it no one ever mentions anything about Richard never aging? You'd think the Others would have realized it, there must be something about him connected to the Island. Why does Dan think his mom was wrong about Jack and Kate and the rest going back to the Island?