I can't answer for someone, but I can still answer your questions, gungonX, here's what I think.
I am positive the memories are fake, because Namine says they are fake. If you want to reread the story, go here Kingdom Hearts: Chain of Memories FAQs - Kingdom Hearts: Chain of Memories Walkthroughs - Kingdom Hearts: Chain of Memories Guides
and start reading from this line in the game script, by dragonblade3325
Naminé: Thank you. Oh, Sora, forgive me... I'm not supposed to be in this
Use Control F to find the point in the Script.
Next, a better way of saying what is meant in the quote is that Sora will be a different person than who he would be had he not had his memories shifted by Namine. Other than that I'm not sure what you mean by until they return the hosts hearts. The time that the memories come back is arbitrary and not the point of the quote nor topic anyway.
Well, the reason why what happens in the game happens in the game is really up to anyone to decide, and can only be vindicated by Nomura. So, if you really care, just read Kingdom Hearts Another Report, and you'll get enough of Nomura's ideas to fill in the reasons for the events yourself.
The reason why Twilight Town is literally connected to Sora is because the Twilight Town that we are reffering to is the one that does not exist. This one, whether physically or not, allows Namine to reconstruct Sora to have Roxas inside him again. So, as one of them started remembering, the other did too, because they were becoming the same person again.
Hope this helps, but your questions were kinda far from the purposes of the text you quoted and are further from the purpose of this topic. So my question for you would be, why?