Interesting analysis. Honestly though, I think you're reading into the changes a bit too much. Over the last several years HBS has had a tendency to change a few essays while keeping some of the old essays from the previous year. The three accomplishments essays, for example has been a mainstay for a long time.
I think HBS sometimes introduces an essay for a specific reason (for example a call for more attention to ethics), but sometimes just introduces a new question to find out something different about applicants.
Overall, I wouldn't remain hung up about why they're introducing a certain question, but rather how to approach it -- which is what your other questions touched on.
To be honest all of your questions about approaching these types of questions are addressed quite extensively in our book Your MBA Game Plan. Nonetheless, I'll try and provide some insights here.
"1. Describe a significant change that you brought about in an organization and its impact on your development as a leader. (400 word limit)"
HBS is all about leadership, so this is a really important question to approach correctly. The thing I would stress is that it is vital to answer the second part of the questions. The admissions committee really wants to hear about what your approach to leadership is and how that has developed. For the first part of question, be sure to specify the impact you had -- don't be overly general.
"2. Provide a candid assessment of your strengths and weaknesses"
As my partner in crime (Scott) mentions, stay away from discussing character flaws as weaknesses -- items that you would have a difficult time changing. Still don't just say "I sometimes work too hard". Pick something that can be corrected and you might want to even go into ways that you're currently addressing it -- you can also talk about ways that HBS would help you address it.
Regarding strengths, don't worry too much about sounding overly cocky. Just mention a couple strengths that you've identified in your game plan that you want to come out in your application. And spend time explaining why you view them as strengths.
"3. How do you define success? "
This is what we call a "personal philosophy" question. The adcomm are trying to get into your head on this one to find out what makes you tick. It's kinda similar to Stanford's "What means the most to you and why" question.
Hope that helps.