I know that in situations like this people commit themselves to "positions" that put them on opposite sides of the fence from each other. And I know that it might seem like you are stuck in a situation where you have to choose one side of the fence or the other. Your mom or your boyfriend, your mom or your baby, etc.
This is your life, though, and you have your own legal rights and your own responsibilities. You're only 16, but you've made choices already that have put you in a position where you have to be mature and take responsibility for those choices. This isn't a romance novel, where you can run off to be with your older boyfriend, the love of your life, and you and he and the baby will live happily ever after. And this isn't medieval france, where your mother can take away your child and lock you up in a convent to keep you and the baby away from your wicked seducer.
The fact of the matter is that you do not currently have the ability to adequately care for yourself and your baby once it is born without support. "emancipation" is fine for young people who can truly do better on their own than they can tied to their parents, but from what you've said here that is not the case in your situation. Even assuming the "best" case scenario and your boyfriend is not jailed for statutory rape and you find legal counsel to keep your mother from adopting your child, if you throw up a wall between you and your mother, you're going to be left with very little choice but to depend entirely on this 21-year-old boyfriend to support you and your child indefinitely. Even assuming his intentions are the most honorable, 21 years old is hardly more than a kid itself, and the fact that he was living with you and your mom for a while suggests that he is having trouble supporting even himself.
Making a choice that subjects you and your baby to the willingness of a 21-year-old guy to support you, and that in the best of situations dooms the three of you to a life of poverty and isolation from your extended family---that's not "emancipation."
most likely, your mother wants to see you make mature, responsible choices. The best way to relieve her fears about your future is to make mature, responsible choices about your present. Yes, you should get legal counsel about your rights regarding your child. Your boyfriend also has rights, by the way, as the father of the child, and he is certainly an adult. So, your mother's plans to cut him out of the picture entirely are probably not based on sound legal advice, and they are probably not thought out very clearly. This actually says good things about your chances of sitting down and talking reasonably about the future of your family.
If you don't start making threats about emancipation and running away and all the rest, but instead recognize the good sense of what your mother is saying--that you need to finish school and accept the support of your family until you're prepared to support yourself--you'll probably be able to come to an agreement that is acceptable to everyone.
By the way, making mature, responsible decisions includes recognizing the mistakes you've made in the past and not repeating them. Planning on shacking up with your 21-year-old boyfriend is certainly not going to reassure your mother that this baby won't have four siblings within three years. If this guy is the love of your life, then you have plenty of time to be with him, and you don't need to make decisions out of desperation that you're going to lose him. Instead, you can focus on what is best for all of you in the long run. And that is most likely going to be (a) getting on birth control as soon as this baby is born, and (b) living with your mother while boyfriend gets his own life together independently, while providing support for both you and the baby and maintaining frequent contact with both of you.
By the way, it doesn't have to be a choice between your mother adopting the baby and terminating your parental rights or you running away and taking on the full burden yourself. You and your boyfriend could agree to allow the court to name your mother the legal guardian of the baby until you reach age 18 or graduate from college or whatever. That won't terminate your rights or your boyfriend's rights as the baby's parents, but it will empower your mother to make the decisions necessary for someone supporting an infant (i.E., healthcare decisions and the like). That might give all of you the reassurance you need.