cdm9999- I'm sorry to read about your situation. I think your concern that your husband wants to avoid the financial obligations he will incur if the two of you were to divorce is valid.
I am not a lawyer, but based on experience and observation, I believe the following are correct answers to your questions.
I don't know about being able to file for a separation. If there is physical abuse, you can file a restraining order which will require him to vacate the premises. If you file for a DIVORCE, I think that based on your current situation, at some point the courts will compel him to move out of the house. Until that time, he will have the legal right to be in the house.
Whether or not you let him build the apartment is up to the both of you to decide. If you build it nicely, and end up divorced, you might be able to rent it out for some extra income. However, if I were in your shoes, I don't think I would go that route. More on that below.
The only way I could see you being gullible and stupid about this is that converting the basement into an apartment will cost money that would be otherwise available if the two of you end up getting divorced. I do not believe that building an apartment will affect your legal position later.
So those are my best answers to your questions. I will now move on to advice!
cdm, it seems like your situation is very salvageable. While your husband is guilty of bad manners and harsh language, he hasn't hit you or screwed around on you. The problem is that he currently doesn't seem to appreciate your good qualities, so he wants "space." For your part, you want him to be nicer to you, and appreciate that he has a pretty good life.
It seems to me that there are some things you could do to get what you want and make the marriage better. But your husband is pissed off and thinks he can set things up where he gets all the benefits of marriage without most of the responsibilities, and avoid the financial hits a divorce would cause.
It seems like the two of you would benefit greatly from marriage counseling. The trick here is to make your husband understand that his life will be really sucky if the two of you divorce. If he understands that, he will want to work on changing things so that both of you are happier.
Ultimatums are a terrible way to run a marriage, so I don't advocate sitting him down and saying "we go to MC or I'm filing for divorce." Maybe approaching it along the lines of "look, we have a good life, and we have some problems, but I think we could be a lot better of we got some outside perspective" would persuade him to go and put some effort into changing.
As far as the basement goes, rather than an apartment, make it into a man-cave! This would let him feel like he had a place to go to where he could "relax," and if he had that, he probably wouldn't even notice when the kids are down there watching TV with him. Plus, it would be a place BOTH of you could go to and watch a movie on a Friday night. Try to avoid having a bed down there- make it where he has to come back up to the master bedroom to sleep.
I didn't spend a lot of time discussing divorce strategies here, because based on what you have written, it seems like, again, your marriage can be re-directed into healthier territory!