K.,
You are doing a wonderful job! Right now you are in the midst of your daughter learning something that she doesn't like: obeying. It WILL improve, just not today.
It sounds like she is phasing out her naps and that is a major contributing factor to fits later in the day. My son (3.5)is in the same boat. Too, she is flexing her little wings to see just how far she can push you and when; as well as in which situations you will follow through. She is needing a little more attention right now. The imminent arrival of another child may be a factor in the back of her mind.
Distractions work well. I also let my son say 'No.' as much as he wants as long as he obeys in a timely manner. I am not going to deal with obedience AND sassing at the same time. He is not quite mastering full sentences (due to being bi-lingual he is a late talker) and I will work on the sassing and backtalk when he can form complete sentences as that, in and of itself is another complete (and on-going)lesson. Sometimes (permission to cringe here..) my son will get so worked up that he will hit me. Oh yes, strong personality. I remain calm and whisper in his ear how he is now going to get MORE discipline. At this point, I will try to detach myself from the situation so as not to take it personally and get emotionally involved. Easier said than done, I know, but it works in remaining calm and objective. Almost as if you are handling somebody else's child.
I am quite convinced that some of you have seen us about Portland/Beaverton. My son will lay face down on the floor in what I have now deemed 'the full worship position', and will cry as if he has been hurt (we all know that cry and it will always snap other mom's heads to attention). On Sunday, I even heard one guy say "Oh man...yikes." as he was cradling his infant daughter. I had to restrain myself from saying "Honey, YOU just wait.".
So be encouraged, be removed from the situation, be aware of possible fear and be consistant. You are doing an amazing job in this difficult time. And should any of you see my son and I whilst in the full worship position, please stop to say hi! :o)