Hi M., I actually put up a post that dealt with similar issues and the advice I got from moms was great and it worked! My post was "Saying hit makes him...hit?" posted a few weeks ago. Anyway, when it comes to the diaper situation, I find that giving choices really helps. It's good that you ask your daughter to pick a diaper - that already sets the precedent that she has some element of control. My son also won't lay on his back to get changed most of the time, and I've learned to try a little of bit of everything, but to vary it up a lot - I will give him a my phone to play with, or a difficult-to-open container (to buy time), or I will sing a song and ask him to sing with me. Sometimes, nothing works, and for that I've learned to change him standing up. To put the diaper back on, I open it up and velcro it together, as it it's already on, and then I invite him to put his "diaper pants" on. Works like a charm.
My son was not a big hitter or biter but he'll try to get on in every once in a while. As I learned from my last post - saying NO or NOT NICE or NO HIT did not work. Instead, I redirect and say "we play nicely." Actually, that phrase has helped tremendously with my son's anger. I ask, "are we going to play nicely?" and I stop whatever we are doing until he answers affirmatively. When he does, then I instruct on HOW we play nicely (we walk, we talk quietly, we hug, we touch like THIS, stroking my arm softly, etc) If he continues to hit/bite/or otherwise tantrum I use my 1-2-3 Magic techniques (that's a great book, by the way, although not so much geared for the younger ones, but it can be modified). If, at the end of three, he's still tantruming, then I call a "together time out" and I take him to the bed or a safe area and let him scream it out while I say occasionally, "yes, your'e mad! I know that you are mad! You want hug, feel better?" While I offer him my outstretched hands. Ultimately, I do find that he will calm down and go for the hug, even if he's still crying a bit. Then, as soon as he is calm, I move on and I don't mention the tantrum or, if he did something wrong, what he did wrong. I just move on. Let me just say, ALL of that was advice I got from the request I posted...thanks, moms! You're awesome!
In conclusion, I find that you have to be creative with the willful ones! If one thing does not work, try another, and don't get frustrated.
I must also add that, like the mom before me, music helps a lot. My current favorite is Laurie Berkner - it actually puts him to sleep! It definitely helps to "break" a tantrum when I've run out of ideas.