Okay, let's deal with the easy question first. Working at a law firm can be very stressful depending on what kind of clientele that firm has. If it deals with very personal casese like divorce, custody, property disputes, etc. or high profile corporate issues, then yes, the atmosphere can be quite tense. Have a look at the movie Devil's Advocate or the Ghosts of Mississippi to have some sort of idea. Working there can be quite fulfilling though.You're exposed to very intelligent people and not so intelligent people also (there are dumb lawyers after all). The bottom line here though is that you'll never find out or have an idea if you don't go there and get to fell it for yourself.
Now for the harder question. Should you go to the interview? I think you should. It used to be thought that guys have to bring home the bacon and provide for the family while the wife stays home with the kids. More and more, that idea is being challenged and women are really proving themselves in the work place so much so that guys get threatened.
Another issue here is that your mother does understand having provided for you before. It's just that she knows how hard it is and she doesn't want you o go through that. It's normal for a parent to think about that without sometimes realizing that her baby is now a woman and is soon to be a parent also.
Stand your ground with her. Parents aren't all that dense though we sometimes seem to be. Sooner or later she'll realize that you have to stand up for your life and your own young family's life. It's actually better that you want to be responsible rather than just be the recipient of dole-outs from her.
A thing to consider though: who'll be taking care of the baby if you go off to work? Maybe that's another factor your mother is thinking of. You should make sure that your baby is well taken care of and that, even if you are tired from work, you are still able to have fun and take care of your baby. What some people do is they allow the husband to work alone for around two to three years and when the baby is that age, then the motther starts working also to augment the income.
Whatever you do, make sure you have your priorities straight: your baby.
Going to work is not a matter of independence or having money for what you want. Working is now a matter of that still but more importantly, it's a matter of having the funds to provide for diapers, formula, baby clothes, toys, medicine, check-ups, etc.
Your mom cares for you and you seem to have a responsible enough partner. It's now a question of how you can contribute positively to this situation.
Take care of your family, pray a lot (together) and you'll be fine.
Erick