Dear D.,
I've seen your posts before and my heart goes out to you, but I have some advice I hope you take in the spirit in shich it is given. You have suffered a terrible tragedy and you are overwhelmed with back to back (to back) children. Your response to something a lot of us have gone through (difficult potty training) is magnified by the constant pain in your heart and the stress of managing time with so many commitments.
I think, if at all possible, you should seek professional help. Looking back over my life, I see how I put a lot of pain and guilt on my kids for things they were not responsible. I didn't do it consciously, but I did it.
Your 3 year old knows there is a child missing even if she doesn't remember him (and depending on the overlap, she might). She feels it every day through you. Will your new baby, born so close to the anniversary of Nathan's death be a psychlogical "replacement" child? Exactly what do you mean when you say you "shouldn't have to " clean up after your not yet potty trained 3 year old and why should a 3 year old "want to grow up"?
Much as you are in real pain, deserving of comfort and having endured a tragedy that will never entirely heal, you are still the parent, the grown up and the kids need to be shielded from your pain. Is your husband supportive or are you basically trying to put up a happy face in front of him? You need someone to lean on and it may have to be in the form of a therapist or grief counselor.
Try to take a little time for yourself every day - even 15 minutes to breathe and gaze at a beautiful flower! Try to share with your husband and find a way to accept Nathan's passing and try not to push these developing years too hard.
There may be a lot of reasons why your daughter wants to pee in her pull ups, but she WILL grow up and the days when you can pick her up and cradle her will be long gone. As a grandmother with 3 grown children, I know how quickly these days will pass.
I really think that if you take a time out for yourself (JUST DO IT - 15 minutes!!) and try to work through losing Nathan you can put some pee on the floor in a perspective that is not as difficult to handle as it is for you now. You are in my prayers.
G.