Book Review: Rise and Fall of the Great Powers
These are great points, thank you for pointing them out. I think I agree with your overall take - the analysis is not finished with Kennedy's framework, rather it's a good place to start. We can then go into more detail on each trade-off - analyzing why Japan gets to an investment rate of 45% whereas the Soviet Union only to 30%, say.
On your specific points:
1. Good point - although I think this can only be taken so far. The Entente powers spent less on the military but had slightly higher overall economic output, and that's why they had an advantage. Certainly its unusual that Germany would defeat France so decisively.
2. I agree! Which I think is one of the reasons why Prussia sits uneasily in the framework. I think its worth noting that whereas France had domestic issues - Prussia had significantly worse external problems. These caused far more destruction than France's convulsions. The "Miracle of the house of brandenburg" and the fact that the non-Russian Napoleonic wars were fought mostly in Germany come to mind.
3. Why do you say it wasn't true pre-Napoleon?
4. I think I have to disagree here. Prussia was in a century and a half contest with Austria the moment it seized the Silesian coal fields. I agree that quick and decisive victories did the trick, but the difficulty is its not clear how Prussia managed to win decisive victories whilst not falling behind economically.