Stop. You can't please everyone. You are in survival mode and need to regroup!
Put your husband first. Children second. Job/School 3rd. Friends last.
If I were in your shoes, I'd go on a date with my husband, tell him that he is #1 and I'm sorry if it seems I've been neglecting him. Let him know where he falls in your list of priorities. If he is #1, everything else will be much easier. If things are rocky with husbands, it tends to spill over into other areas.
Kids are little once. If I were in your shoes, I'd wait til my kindergardener was in full time school to carry 6 or more credits again. Here it is usually first grade for full time school. Your classes will still be around next spring/fall if you decide to spread classes out a little more thinly or temporarily suspend them. Your baby will not.
You said your husband has a very decent paying job. I would cut down on my hours at work. Are you able to do 2 days/week? That would free up quite a bit of time if you can afford the pay cut. I would do it. Money cant buy a happy marriage or moments with your kids.
And I know it seems impossible to find time to do this, but are you excerising at all? It'll help your body and your spirit gain more energy.
And finally, the inlaws. Your family seems somewhat traditional and I dont think youd find it offensive if I assumed he wants to be 'head of the household' and youre OK with that. Since he'll be your #1 priority, you should also be his.
He should, and may want to, take the lead and control his parents when they go too far. As his wife, you come first and are above everyone else, including his own parents. You are his priority, he loves you and wants to protect you. He should be able to calm the seas with the inlaws when he sees them starting something. They will learn how far they can push it.
When spouses put each other first, the family stays strong. To get through everything, you need this network.