I cannot speak to what your lawyer told you but if your fiance is in the USA he or she does not need a fiance visa . You do need to get married to sponsor your spouse. The filing fees are $1490 if your spouse is in the United States. They will get work authorization approximately 3 months from filing the application and the green card in about 6 months. Cheap sometimes is expensive. Go with an attorney who has good reviews and is recommended.
You can leave the USA at anytime without a problem. As for your husband, he will have to satisfy the immigration laws of your home country. He is also free to leave the USA and come back as he likes if he is a U.S citizen