First a book recommendation: Kids, Parents, and Power Struggles
by Mary Sheedy Kurcinka. It's not specifically toddler oriented but helpful in an "overall philosophy" way.
Power struggles are the first sign that your wonderfully intelligent child is developing a good strong and healthy will. I've found that by having as few rules as possible and keeping them age-appropriate, helps mucho.
DS is an on again, off again diaper struggler. When he's on, it can be so tough! I remember when he was younger, I could give him a toy or even a usual "no-no" (my glasses for instance) and this would keep him occupied while I "fast as a whip" got the change done. Now that he's older, I find it very important to engage him verbally and playfully while changing the diaper. The "where's your nose, where's your eyes" game works well usually or we sing songs he knows that maybe he's learned a little finger play too (itsy bitsy spider). I find that if I stay happy calm and engaging, he's much more still and able to handle the change. As with so many other things that come up, my happy go lucky attitude (no matter how forced on some days ), can really turn a situation around.
For your second instance, I think I would focus less on people's "feelings" at this age. While certainly, it's time to set the tone, 19 months is pretty young to understand and respond to people's "feelings." I would focus more on hurting people in a physical way which is something that he definitely *can* understand now as I'm sure at 19 months, he's taken his share of spills. What I do exactly in this scenario is this: I tell DS that he's not allowed to throw his blocks at people because he could hurt someone. I then tell him that if he wants to throw something, he may throw a ball gently in the house or outside (I show him to throw into an open space rather than at someone, something or a pet). This redirection usually works quite well for us however, there have been times when I've had to place the "item in question" up for a time, but most of that was early on. Anymore, he responds to *pointed* redirection--has to be pointed by the way, general redirection (to another type of activity), rarely if ever helps.
Another thing to remember for us, is to address any underlying emotion that may be involved. If he gets really worked up around certain people (a special fun loving Uncle or Auntie say), he may be just overly-excited. Or if he's angry for some reason and throwing things. In each of these instances, I would add "You are feeling *insert appropriate emotion here.* It's Ok to feel *said emotion.* But, it's not ok to throw at them, because they could get hurt." By addressing and validating the emotion, your child knows you're tuned in and not just handing out the standard line. In other words, he usually more ammenable to working with you.
Sorry for the novel here... I hope this helps a little. Hang in there!