Did you ever get test results? Do you think the test was reliable?
Maybe #1, not #2, possibly #3 if you have counseling and talk about the situation while a counselor is in the room, not #4.
I take it from the way you talk about her that the mother is not officially your girlfriend or fiance and that you are more interested in being a dad to these kids than you are in her as a partner? If that is a correct assessment, and if she understands that, then if the DNA test turns up negative, just tell her you would like to be in the children's lives, and define in what way you would like to be around. An uncle? A friend? A dad who doesn't live with the family?
If you would like to marry the mom, then it is a more serious matter if the test comes out negative, since you would then have to deal with apparent infidelity. (Or were you not in an exclusive relationship?) If you are trying to have a long-term relationship with the mother but also have the complicating factor of thinking she is unstable, if you find out you are not the biological dad you do need to deal with the overhanging suspicion that she has had a child with someone else's DNA and has implied it is yours to you and accepted your support all through the pregnancy. For this, you should probably seek relationship counseling together. It would also help with the fact that you characterize her as unstable.
But what do you really want in terms of a relationship with the mom? I'm not really hearing you say you want her, just to be a dad. Sticking with a woman for her kids is not really a bad thing, since the kids benefit. It just shortchanges you in the future when you meet someone you really love, and shortchanges her also.