Unless he's aware of how childhood friends usually are(via manga or just other kids around them)...and knows her actions toward him are unusually aggressive.
How did she know he was rich before going to his house? Ryuuji wasn't the type to flaunt his money. Moreover, when she explains her reasoning, her words imply that she just found out he was rich("so, you're family seems to be rich, too" or something of that extent...it's also said in a way that...
Some kids are afraid to go to the bathroom by themselves...especially in the middle of the night when the house is dark and spooky.
After how much Nana wanted Ryuuji before...you're surprised she still wants him? :/
So was Nana's...there's no way she could have known his family was loaded before going to his house. Him being rich(though he doesn't seem all that rich now...seeing as he has to work for a living) was just icing on the cake to her...a pleasant surprise.
Like I said before, it's made clear how Mana takes after her mom...her mom was smart enough to only go after Air-kun on her safe days until she was ready to get pregnant.
No, that chapter said it had been a month since she found out she was pregnant. We don't know how long they were going at it before then. Up to that point, she had been attacking him on her "safe days", is the only thing we know.