"I just need some answers regarding legal issues so I can make some smart choices for my own life."
First off, I'm not an attorney, and your best bet is to speak with an immigration lawyer.
But I do have friends who have married immigrants, so I have seen the Visa / Green Card process for Americans who marry immigrants close up.
If you file for a visa or a green card based on being married to an American citizen, the Immigration and Naturalization Service (INS) will investigate thoroughly to make sure that you aren't in a sham marriage. One of my friends, who married a French woman, was taken into a separate room and asked details about the birth control that they used, and what color her toothbrush was, to make sure that his answers matched up with hers. He forgot the color of the toothbrush, and their application process was delayed by many months while that was cleared up.
If the INS uncovers that your husband is spending his days in a motel doing heroin with another woman (and most likely they will), this won't help your visa process at all. It may help that you have been married to him for a long time, but expect that a whole host of questions will come up in the process that you will have to deal with.
Again, probably best to speak frankly with an immigration lawyer so that you know what your options are here.
What concerns me more is that it doesn't seem that you are seeing the situation with your husband clearly. On the one hand, you state,
"At present he's currently holed up in a sleazy motel with this woman slamming as much heroin and meth as they can manage all day every day. (he left rehab about 2 weeks ago) Asking for money off of everyone in his family. It's so sad."
But then, you say:
"Any advice on how to talk to him to make him feel safe? I feel like he won't talk to me until she's not in the picture coz I think he's just stressed out to the max. All the emails I've sent him have been kind, understanding, non-beggy, and letting him know that I know he's going through it right about now."
He's holed up in a hotel shooting heroin all day and your priority is to be understanding and non-beggy? You want advice on how to make him feel safe?
What about you? Don't you have the right to feel safe? Don't you have the right to a husband who is there for you and who doesn't go off and spend his time shooting heroin with another woman in a cheap hotel room?
You have invested a lot of time in him. It's hard to walk away from the gambling table and accept a loss. But he's lost in active addiction. And he is taking you down with him. There is no telling if or when he may improve. For many, addiction leads to institutionalization and death.
How much more of your one precious life are you willing to invest in this man?