(1)I'm not a doctor, but yes that absolutely sounds like depression, so definitely talk to your doc, maybe even see if you can get an earlier appointment if you can't take it anymore. Get online and read about the best way to treat depression - I seem to recall reading that a combination of therapy and medicine are best. (Start your online search with wikipedia - they usually have good, unbiased medical info.)
If not for yourself, take care of it for your baby and child. You'll be able to be a better mother if you can get past the postpartum depression. It can be dangerous - I don't want to freak you out, but... you've got to take care of yourself, too, or you won't be able to help anyone.
(2) Good, fresh healthy food should help your weight and your mood, at least some. But try not too come down too hard on yourself right now. More fruits and vegetables, less meat and cheese. Turn off the Paula Deen show, and pick up a magazine with fresh, APPETIZING healthy food.
Vegetarian Times or something else that looks healthy.
(Assuming you have the energy to cook - if you lack the energy, you can try the frozen food section of the health food store to tide you over.) And get a little sunshine, preferably while talking a walk or while taking the kids to a playground.
I'd also recommend taking a fish oil (DHA)supplement,which
has been shown to help people suffering from all kinds of mental issues, PLUS it will make it's way into your breastmilk for the baby (GREAT for building baby brains). It wouldn't take the place of an anti-depressant prescription, but it would just be an added benefit to your diet, and it's also good for your heart. (Don't take my word for it, though, do some online research.) It's very safe - I used to add fish oil to my baby's bottles of pumped milk when he was a newborn. (It was strawberry flavored!) And my doctor OK'd it.
(3) New babies almost always disrupt the toilet training skills of the older siblings, unless the big kid has been doing it a looong time. We went through potty training hell when our second came along. I'd advise you to completely back off, let him go back to diapers or whatever. Just avoid the power struggles or things get more drawn out.
Besides, you've got WAY too much that you're dealing with right now, you can't do it all. If you have to start potty training all over again in a few months, so be it. It sounds like that it's the least of your concerns. Put a waterproof cover on the sofa for a while, until you get through the PPD.