The whole thing sounds fishy to me too. She has been dishonest about the dating site and she disappears each weekend. I do not see great things here and I know you have emotional ties, but I don't see anything productive here or a real honest future.
You are ready to start a marriage with someone whom has yet to be honest with you...this is cause for red flags. Has an answer for most things...trust your gut here and your gut screams warning.