Question 1:
1. Yes, he will lose his house when he moves in with your sim. Be sure to click "Sell Furnishings" when he moves so he'll bring more $$$ with him.
2. When you sim breaks up with him, you can go into Manage Households to "Create Household" for the ex-NPC to separate the two households.
Next, you can move him back into his old house. He will start with $20,000, though. If his former house costs more than that, use the "Freerealestate on" cheat.
(I'm assuming you're playing a for-fun game and not a challenge where cheats wouldn't be allowed.)
Question 2:
For building tips, go to Carl's Guide's "Gameplay Info" tab and look at the
Building Homes & Lots category.
1. It's really a great idea to take a good look around Carl's Guides.
Under the "Gameplay Info" tab, you'll find the
Moving Sims page.
It explains how to create a new household which includes the sims/pets you wish to split off.
In Manage Households, click on the heart icon for the girl's household so the game won't cull them.
2. If you move your new household into a house, then yes the guy can visit his son. And he can also invite the other household to visit his home, whether the new household lives in a house or not.
About the girl, if she lives in a house, then yes you can have her visit the twins.
Good luck and happy simming.