Why do you only ask about "large points"? If you implement 'most of CSS, except for minute details', then you haven't implemented CSS, and web developers suffer because they have to add UA-specific hacks for your implementation.

To answer the first question: The first step would probably be releasing more often than once every 1.5 years.

Unfortunately, MS considers IE a Windows component, so MS supports all versions of IE that was ever released on a version of Windows for the life of that Windows version, which is a minimum of ten years total for each version.